Thanks, Don.

Between having played music for 25 out of my 29 years, and having had several arm-stressing jobs, I find myself paying a lot of attention to my wrists.

I play standing up, but it's BECAUSE I can't keep the guitar in a comfortable position sitting down, and having it on a strap lets me get it just right. Probably depends on the size of the guitar and size of the musician, though. I also keep it hitched up just a bit high--a lot of the younger rock and C&W musicians around here seem to think it looks cool to have your guitar around your knees, but my left hand would break off if I did that for a extended period of time!

I avoid A and D chords like the plague because they make my hand cramp up after awhile if I'm playing fast. The capo is my friend. (Also, since I'm a woman and smaller/shorter-armed than most of the [male] guitarists I know, this lets me play just a little higher on the neck, which further reduces the need to bend my left wrist.)

My record for longest playing session is five hours, at a dance. My wrists were fine but the calluses on the fingertips of my left hand were long gone.