There have been times I've pieced standing up. It's only been when the stitching is very short segments with other activities (pressing, triming, designing, finding stuff) in between. It was a comfortable position rather than sitting then standing repeatedly. I didn't stay in one position very long.
I've heard/read that when standing for long periods (washing dishes) without much movement, resting one foot then the other on a small block actually helps the back. I'd consider this statement as true as gossip, so do some research before quoting.