I run a PT Pyro that is half DG, half campaign, and is optimized.
Since I do not have a threat dump, I always ask for guard at the start of the FP if the tank appears to be not as geared equally.
I always feel like a jerk asking for it, but I know if I don't get it, I'll pull off the tank. Like I said though, if the tank is geared, I won't ask. But I've been told numerous times by tanks that they want the guard on the healer. Oh well.
DPS shouldn't feel bad for asking for a guard. You are only doing what you think is best for the group. DPS should always ask for a guard if they are unsure the tank can hold threat. I play a PT dps as well. I'm gonna request a guard to any tank, because I know there is no possible way the tank is going to keep up with my 3000 dps I pull out in the first 15 sec or so of a fight. With that guard, it means that I can go all out, which is very advantageous to the group. Don't think it as boasting or cocky behavior. You are only doing what is best for the group.