I can recall a quote from the late Dimebag Darrell.... talking about how he writes all his riffs standing up... "You don't go to war sitting down"

I practice standing up a lot just to keep my muscles use to it... If you've ever watched Dream Theatre... you might notice how John Petrucci always puts his foot up on something when he goes into a fast run or a difficult passage... this is becuase he was classically trained and classical guitarists always sit down. This helps to stablize your whole body and allow you to concentrate on just playing guitar... I agree with Dimebag though...