Because of the size of the stands that we have in our woods, I generally stand, and I stand for most of the time that I am in the stand too. Lately however, due to a broken leg/ankle i've been shooting while sitting out of a ground blind, and I shoot every bit as good seated as I do standing.

I guess it depends on the situation for me also, ive shot severaldeer sitting and standing both, but if i had a choice i prefer to sit and shoot.

If a deer is on the move and I will have to change position alot I will stand but if it is slowing working into a lane and calm I will remain seated. I have taken deer both ways.

Sitting ,My legs dont work

I've shotsitting, standing, leaning out, leaning right, left, bending around the tree. Heck, I even killed one deer while hanging out around my stand by my safety belt (feet on the deck, torso leaned all the way out to the left, suspended by the belt, and shooting back behind the stand).

I practice sitting, standing, standing on a bucket, turned awkwardly, etc.
This will also help get your heart rate up for your practice shot, and increase your nervousness. It helps you "learn" to beat the buck fever, IMO . . . or at least deal with it a little.

You've gotta trust yourself and your gear. If you're only comfortable shooting while standing, then stand. Be careful if you stand, make sure you can do so silently. Some stands creak, or you might have a knife on your belt that could catch on the treestand railing, etc. Tie up all the loose ends.

standing, unless I am caught off guard. I feel that i have more mobility and control over what is going on.

Always standing. I do a great deal of 3D shooting and that is just what I am used to.