Whether standing, sitting, or even lying down, one's shooting frame should be unwavering and unchanged. Your shooting frame is like the foundation of a house; everything builds off of it, and your success in shooting depends on it.
If you are interested, in the video below I discuss the importance of utilizing a solid "T" shape shooting frame and how to use your frame to shoot in different positions, from a tree stand or sitting down. (this is later in the video)
Now I actually made this video to help compound shooters who are interested in trying out traditional archery, but archery is archery and whether you are shooting a compound bow out of a tree stand, or a tricked out recurve on a 70 meter Olympic line or shooting ducks out of the air with a longbow, the shooting frame is a shooting frame. All archery requires the same fundamental principles. So in other words, sitting, standing, etc., really is no different from one another. It is only different when one collapses their frame. That's when shots are blown.
https://www.youtube.com/watch?v=ebS6YPG1Q5k
Also here is a photo of John Schultz who is shootng a longbow laying down on his stomach. For fun I turned the photo upright so that you can see even though he is laying down, his shooting frame is still a solid T shaped form unchanged.
https://imgur.com/pEXqJR8
https://imgur.com/Lv6MWo9