I've never noticed that mine drink after they eat, but I would be inclined to think they would be less likely to at that time. They will drink occasionally, but BPs don't drink like mammals do because they are so much more efficient at preserving water. If your BP is thirsty, s/he'll drink. It's probably that s/he does drink, you just don't see it. Just keep clean water available, they'll take care of themselves.