Since a snake eats and poops ~10%+ of its body weight easily, a 10%+/- difference in weight doesn't mean anything at all. If you do weigh (not necessary, really) weigh when it is convenient for you and for the snake, and write it down, and over time (months/years) the patterns may be interesting, or not.

Small changes in weight up or down aren't worth thinking about, since they're just noise.