I thought it was SOP to warm up one's voice before a performance. It seems to be common sense. If stage lights are involved in a performance, the stage area is made somewhat warmer by them, so often people will leave their stringed instruments in the area so as to acclimate the strings. Heat expands things and cold contracts them. Much the same with vocal cords (which are muscle tissue). It also prepares the voice to be agile without straining the cords. If one's cords are still stretching while one is singing, then note control becomes much more difficult and requires more 'push' from the chest area as opposed to the stomach area, so warm ups should be part of the pre-performance ritual. If there is no area/room for the singers to exercise their pipes, then use a bathroom. Besides, bathrooms make us all sound better ;-)