I think that it's really a question of how well you're doing in terms of survivability. In other words, how well the healer is keeping up with the damage you're taking on this particular encounter. If you're *really* close to the edge in terms of survivability, then you probably want to skip Assassinate/Spinning Strike so that you can continue getting your FL/TkT as fast as possible. A proc'd FL/TkT does a huge amount of damage in any case, so it's not like you're failing to contribute to the DPS bottom line by not using your execute ability.
With that said, this situation is the exception, rather than the norm. I can only recall one or two scenarios where I had to forgo Assassinate/Spinning Strike in order to keep my healing up. In nearly every case, I will do what Kitru does, which is bump it to the top of my priority list (basically, using it on CD).
which is why you don't use DS in the pre-execute thrash-less (not enough damage to justify the cost).
Strictly speaking, this isn't true. Double Strike's damage-per-force is really good, and quite a bit beyond something like an unproc'd Project. The problem is that it delays Telekinetic Throw, which is the *real* heavy hitter. If it weren't for that, it would be a core component of everyone's rotation. Spinning Strike has the same problem, it's just that the damage is quite a bit higher.