As a sharpshooter:

never

-Quick Shot

-Headshot + Cheap Shot

-Blaster Whip

almost never

-Sabotage Charge (sometimes use it during Burst Volley)

-Surrender (only when I'm bored, tanks have enough taunts (which are off gcd anyways) to hold aggro without using it)

-Sweeping Gunfire (only during objective defending in PvP when I'm bored sitting there doing nothing)

Dirty Fighting (actually my lethality sniper though )

-Quick Shot

-Blaster Whip

-Headshot + Cheap Shot

-Charged Burst

-Surrender

-Sweeping Gunfire

-Smuggler's Luck

I never use Cheap Shot, but that's it. I have everything else keybound, although certain abilities are far, far less common. Depending on your play-style, there are things you won't ever use, but as a "well-rounded" endgame player I have found various uses for pretty much everything.

The post above is generally correct, and you'd be fine never using said abilities. However, if you are curious, some situations I find those abilities useful are:

Quick Shot: Any time you're full energy and moving for a prolonged period of time. In PvP, it's almost never worth it. In PvE, I've found it particularly useful if you're targeted/pulled by a Hypergate during p1 HM TfB. Even if the rest of your DPS is slow to target it, you can (very quickly) pop cover -> sab charge, and then Quick Shot, Flurry, and Quickdraw to kill it yourself, and still be full energy by the time you're back in position. Any other time you're more or less better off just using Flurry.

Headshot: Only in PvE, obviously. Most useful while solo'ing dailies and heroic 2+'s. It does a decent amount of damage for no energy cost to stunned targets (including any you might have Guss CC'ing), so it lets you burn your energy a little bit harder the rest of the time. If you don't do dailies or solo content, the only Operation fights I can think of off the top of my head where I have used the ability is on red adds on Bonethrasher or after stunning the Trandoshian add that casts Deploy Shield as a DF spec. As MM you'll have far better abilities to use, and more than enough energy.

Blaster Whip: Very, very rarely used this in PvP against a Scoundrel/Operative that have Dodge popped while my tech abilities were on CD or not applicable. Makes for a funny kill, but you wouldn't ever miss not having it.

Sab Charge: Even as SS you should have this keybound for burst or filler. It does more damage than Charged Burst, assuming you're using a basic spec with 2 or 3 points into Independent Anarchy.

Surrender: Really no reason not to use this. It's off the GCD and can be very helpful in endgame, depending on gear discrepancies or at min/maxed levels of gear. In a straight 1v1 "threat" battle, an unguarded Gunslinger can almost always pull off most tanks of equal gear level unless they're working on adds or such. Near the end of the fight you shouldn't ever have to use this, but after your first rotation you should always pop it, and sometimes again on new adds or threat wipes.

Sweeping Gunfire: Like was said above, worth it for PvP Objectives and not much else. The damage is too little for its energy cost. However, you can use it to fire instant shots "around" the backside of a Civil War turret where you'd normally be out of LoS, making it infinitely valuable on that map.

Charge Burst/Smuggler's Luck: You'll never have to use this in a general rotation as DF, but very rarely it might be worth using for a burst "non-DoT" rotation. Depending on energy constraints, I'll occasionally do an add rotation where I can kill something using Speed Shot -> Aimed Shot -> Smuggler's Luck -> Charged Burst -> Quickdraw to kill it. Anything with more HP and you should use a full DoT rotation, and anything less is better to just use Flurry if needed. The only fight I can think of where I actually do this every time is to pop Ciphas' shield on HM TfB Council, and only because our DPS is high enough that I don't need to DoT. For anything else, there's a better ability.

Never use:

- Headshot, Cheap Shot

These two abilities are useless both in PvP and Ops, so using them in the solo play is detrimental to rotation practice.

- Blaster Whip

Somehow never got to use it

Don't use as Dirty Fighting:

- Quick shot

DF is so much energy-dependant that the filler must always be Flurry.

Use as Dirty Fighting circumstantionally:

- Charged Burst with Smuggler's luck (pvp)

If everything is on CD, and I need burst

- Sweeping Gunfire (pvp, solo, trash-killing)

If more AoE needed

Use quite often:

- Surrender (pve)

Why, after AoEing a big trash group with Flyby, TG, Shrap bomb, Sweeping Gunfire, TG - I always need it and fast!

- Sab Charge (pvp, pve)

Good damage and burst

Dirty Fighting (actually my lethality sniper though )

-Surrender

Then you're doing it wrong, stick to your Commando.

But in all seriousness, I never even trained Headshot and Cheap Shot because you never use abilities like that once you're 50. If you need an extra boost or are having troubling soloing then I could see grabbing them, but other than that they're a waste of credits.

Everyone else already said most of the ones for DF and Sharpshooter, so I guess I could bring PvE Saboteur into the mix.

Never:

-Blaster Whip

-Leg Shot

Rarely:

-Aimed Shot

-Charged Burst (too much energy in an energy heavy rotation)

-Flashbang (AOEs wreck ALL the CCs!)

-Sweeping Gunfire (only when a lot of AOE is necessary)

-Quick Shot (if Hot Pursuit is spec'd, otherwise never)

Other than that I get around to most of the abilities because of how flexible Sabslinger can be.

I never even trained Headshot and Cheap Shot because you never use abilities like that once you're 50. If you need an extra boost or are having troubling soloing then I could see grabbing them, but other than that they're a waste of credits.

 

Everyone else already said most of the ones for DF and Sharpshooter, so I guess I could bring PvE Saboteur into the mix.

 

Never:

-Blaster Whip

-Leg Shot

 

Rarely:

-Aimed Shot

-Charged Burst (too much energy in an energy heavy rotation)

-Flashbang (AOEs wreck ALL the CCs!)

-Sweeping Gunfire (only when a lot of AOE is necessary)

-Quick Shot (if Hot Pursuit is spec'd, otherwise never)

 

Other than that I get around to most of the abilities because of how flexible Sabslinger can be.

I don't know, I'm a 50 PvE Sab and I use Leg Shot, Blaster Whip, Head Shot, Cheap Shot and Dirty Kick enough that I wouldn't not have them on my tool bar. That's the nice thing about the cover bar, if you get a pesky boss type that kills or incaps your companion or a group of mobs where one breaks away and comes after you all you need to do is stand up and you drop the cover bar. That's where I keep my non-cover abilities, I find all of those very helpful because they do decent damage if the mob gets right up on top of you. And sometimes I just like to run up to a mob and kick them in the junk and smack them to death because it's fun.