I will second what Stephanie said -- I use the same temperature as I do with regular shirts. (Mine is 330 for 20 seconds, peel tape, press another 10.)
I have pressed some burnouts with heat press vinyl as well as rhinestones and the vinyl was kind of......wavy. I called the vinyl company and they told me to press the shirt for a good 10 seconds or so right before doing my stone/vinyl press, so that the shirt will do its shrinking from the heat before you place your design. I think this is more important for heat press vinyl than rhinestones, but it certainly won't hurt to do it for the rhinestones also.
And like Stephanie said, make SURE to put a teflon pillow or sheet inside the shirt, or your stones are going to glue to the back of the shirt and tear up the shirt when you pull it apart. Guess how I know this?
I want to finish by saying that rhinestones look FABULOUS on burnout tee's.
