I still say that the instructions on rehydrating yeast in warm water that some sources have, are incorrect. I believe that the use of warm water invites the possibility of overheating the yeast and killing or injuring it. If I rehydrate, its only in room-temp water.
The regards to rehydration, there are plenty of scientific trials that show that rehydrating improves the viability of yeast. However, improved viability doesn't mean that will make a meaningful difference to your finished beer. It seems to make little difference based on my experience, but I do rehydrate when I have the chance.