maybe if you're going down a 1% grade for 100 feet...but if you're moving down a 16% grade at 50mph standing up would not be so smart. Descending down any serious grade involves serious attention and strength. I've had cars pull out of driveways onto a road ahead of me requiring me to use all my strength to push my butt back while I braked as hard as friction would allow. Standing would be a disaster. So if the hill is a little baby hill and you really, really don't have to worry about braking you might get away with it but personally I'd be giving my butt a break by standing when I am climbing.