I flip up my M1A buttplate and use it standing, sitting and prone. It just seems natural to me to use the rifle "accessories" to help me shoot it. I've often recommended this to other shooters when they are having problem in the standing or sitting position. Most try it, then go back to not using it. Seems like most folks have the rifle butt very high in their shoulder pocket (maybe 1/2 of the butt sticking up above their shoulder.) I just realized yesterday while out dry firing my M1 in the back yard why it may work for me and not for some other folks. I have fairly long arms, neck and big head. I can get my M1's buttplate all the way down in my shoulder pocket (standing position) and "turkey neck" my cheek down on the stock and actually touch my nose to the back of the receiver comfortably - I'd never fire it that way - I've seen what happens to folks who try it with the M14 (old service story I'll tell some other time).
It's up to you, do what works for you, using the butt plate works for me.
My favorite trick with the buttplate is to apply safety tape (3M tm Outdoor Safety Walk) to the buttplate to prevent it from slipping off of my shoulder. It's a bit tougher with the M1 since you have to cut out a line to allow access to the butt stock compartment.
I picked up a roll of it for probably < 5 bucks a couple of years ago and haven't run out yet between my rifles and a friend's too.
Cut a long enough length to cover the buttplate, trim around the edges with a razor blade, shoot, enjoy.
Gromit