I almost never weigh myself. I can generally tell if I'm gaining too much weight by the way my clothes fit me. However, it makes sense to weigh yourself after a hot and hard workout to insure that you are replacing your fluids @ an adequate rate.

If you're interested in tracking your weight loss, make sure that you weigh yourself around the same time every day. Depending on a variety of factors (hydration being one), your weight will vary throughout the day.