It depends! For wheat beers I have mostly just sprinkled on top of the wort and been trying to do this at the same temperature as the fermentation should undergo. For the more white beers, e.g. largers or IPA´s and APA´s I would strongly recommend to consider to both re-hydrate and ensure the hydrated yeast is added at the fermentation temperature. For advice on re-hydrate please see more topic in the FAQ!
For a more Scientific understanding of re-hydration please see: