Like CF said, for Wii Sports you do far better, and it is actually far more fun, if you stand up and try to get into the games properly. For the other ones it just depends on what game you are playing and what exactly you are doing at the time. I seem to vary between standing up in action sequences while sitting down if I am just strolling around in Zelda or something.
Also agreeing on the gym membership, unfortunately I found that out about two weeks too late
