Stand when possible.
Try taking a shot to your right while sitting. Go ahead, pretend you have your bow drawn as you are sitting there reading this and do it. Bow arm out, release arm at proper anchor, full draw, and elbow back. You are lucky to get little more than 90 deg of the 360 deg around you. Standing lets you twist around or silently take mini foot twists or steps to make that shot to the right when the animal presents the shot there. Even with feet planted, you have far more range than when sitting. Stand and see for yourself. My last challenge to you is to shoot a few groups when sitting and compare them to when you are standing. If you are honest, your groups will be better when standing. Practice for standing AND sitting shots, but hope for standing.
The negative part is it is harder to stand still than to sit still. I find myself sitting the better part of the sit for that reason but when the activity going on, I am trying to stand.