I don't think that it matters when you weigh your snake, as long as you are consistent. Consistency allows you to compare one weight to another.
When our BPs were small, we weighed the day before each meal (we were quite obsessive). Now that they are older, we do as others have suggested and weigh after each shed (which usually is accompanied by a nice pee and poop).
The only exception is when they have fasted. Then we weighed every two weeks to make sure that they weren't losing too much weight.