Like David/Dark Knight, I learned to play and practice sitting (55 years ago). When I started my comeback, I decided to try standing to see if would help with posture and breath control and now I always stand during practice at home. The band I play in does everything sitting so I have a direct comparison. I think that standing has helped my breathing but I find that I can play longer without tiring when I sit. So, it seems that there are advantages both ways. I think it depends a lot on the individual and how one approaches it.