New tile floor over concrete slab?

I'm thinking for remodeling my downstairs bath and hoping to start in the next month or so. I plan to put down marble tile on the floor. My house sits on a slab with radiant heat. My concern is, if I don't get started relatively soon, will I be able to put the new tiles down when the weather gets colder? I've heard that you could not tile over radiant heat in the winter, as it will affect the thin set and dry it out before I could lay the tile down on the floor. Is there any truth to this?