really? my exotics vet said it was ok.
In all the research I did, I literally came across hundreds of different books and websites that always said never to use a heat rock.
I think the only reason people even use them with lizards is because heat speeds up their digestive process and most of the foods used in captivity are a little harder for the animal to break down compared with the wild, so adding the extra heat from the rock will sort of equal things out. That's not an issue with a turtle.
If you're using an overhead heat source, you're still going to get a safe amount of heat stored and radiated back up by the substrate in the terrarium, just like it would by the sun. I don't see any real reason to amplify that with a powered rock.