Hi Rebecca:
Actually, there is a case to be made for both methods!
I have a tent, and I have found that I can safely drive my temperature higher for longer periods of time with my head outside... plus, with a giant fan, you can use your head just like a radiator to cool the body, making it easier to manage extreme temperatures... even so, you CAN still drive your body temperature to dangerously high temperatures.
When doing serious sauna detox, I always monitor my internal temperature, both to push myself past imaginary limits, AND to be certain I don't push myself too far.
It is actually safer with the head outside of the sauna, but it is "non-traditional". I, however, have yet to find a "down side".