Hmm, I honestly don't think about it anymore... I just do it on instinct, I guess! Now that I think about it, I generally squeeze when I'm sitting down, as opposing to standing up, just because it's easier... but I'm sure either method works well.
I don't leg on with every step posting. That will totally dull the horse out. I put leg on if the horse is falling out of rythm, and I need to freshen him up,I put my legs on quickly and with a kind of "tap, tap" with my ankle bone against the horse's side. I expect him to surge forward. Then I won't put any leg on at all , until he starts to do something I didnt' tell him to do, like fade out again.
I think I do it when seated, but it's so automatic, you know.