for the past 4 months i've been working on losing weight, both through riding and through changing up my diet/eating habits almost completely. So i've been stepping on the scale every morning after my morning *ahem* "business". I don't really trip out when i see my weight go up a pound or two, so long as it's only for that day. If i were to see it go up 2 pounds and stay that way for 2 or 3 days, then maybe i'd start to think, "ok, time to get strict about my diet/exercise again."
Also, for the past couple weeks, i've been weighing myself before and after my long weekend rides to make sure i'm hydrating decently, hoping to not see any change, but really trying to make sure that i'm no more than 2lbs lighter after a ride.