The packaging says to put it in 100 degree water, so I do. I follow what the packaging says because it is written to give you the best results, not the worst.
I notice that when I rehydrate the yeast, it starts activity a few hours sooner than if I hadn't. But if I buy something like pasteurized juice for the millionth time that I already know the SG and pH for and I'm not adding any sugar, since I'm not handling the juice at all, not even to sulfite, I'll pitch the yeast straight in to reduce contact. It will eventually start eating.
Strangely, my husband was working with a beer yeast last night that said not to hydrate it, but he decided to, so it got glopy, and the glops are now floating on top of the batch.
Point is - follow the directions on the packaging to create optimal conditions for your yeast, which yields happy yeast and a good ferment.