If your cage has a number of various thermoclines in it, and the snake has a choice of what temperature he wants to be, then let him do it. Snakes tend to feel more secure when they are jammed into tight spaces, so perhaps that lip is comforting for him. Give the snake choices, as long as none of them are dangerous, and then just let him choose what he likes.
In theory, anyway. It doesn't hurt to be a little bit paranoid and believe your snake might be suicidal at times and your job is to prevent that from happening. Snakes get along just fine out in the wild without us mother hens looking over them. But then on the other hand, some like to lay on roads and fall asleep to be run over by the next passing vehicle.
How is that for a confusing answer?
