Mine always start on the cool side at the front most part of the tub... That NEVER lasts long, The next time the tub's opened the bowls been moved every which way.
I keep mine on the cool end near the opening of the rack so they're easy to refill. My snakes don't move them so it's quite convenient. It's not really important where you keep it unless you have humidity issues, but I like having it at the opening so I can just slightly open the tub for a quick refill.
I keep mine on the Warm end. I have my tank setup weird lol. I have the Heating Pad on one side and then the Lamp on another. But on the outside of his hut it stays cool so he has a Cool end and a Warm end.