I usually use a strap, standing or sitting, for the reason Bill gave as well as for the fact that your hands will be free to let go of the guitar should you need to. Having both hands free to reach for something is a good thing during practice sessions, and for song sessions, it's good to be able to catch a can of brew without causing needless delays, heh heh.

Seriously though, as was pointed out, you won't always be able to sit down. I try to mix up my practice between the two just to stay familiar with the different feel of each. If you always sit, or always stand, it will make you a bit nervous when it comes time to do the other.

Straps are a necessary evil I guess. You can't play very well standing without one.

Clifton53