I don't see the point in making the room hot to ease your muscles is better, as the aim is to stretch your muscles in an "easing" way anyway. It seems as though that kind of approach means that all yoga is about is becoming more flexible.
I agree with Nadia here. Yoga should be able to be practised regardless of the temperature. The aim is not for an easier stretch (or else one wouldn't be encouraged to yoga in the early morning) it is to spread conciousness throughout the body. Alignment and quality of consciousness is what to look for in asana.
Is this Bikram Yoga that you enquire about?