I prefer to rehydrate dried yeast before I add it to a well thrashed wort. For instance, Danstar recommend you rehydrate their Nottingham yeast in 100ml of 30 - 35ðc water (boiled, then cooled to the correct temperature) and leave it for 15 minutes before adding. If I'm using a liquid yeast then I always make up a starter, to get the desired cell count. Whichever yeast I use, I tend to check (and check again) any instructions that come with it. Mind you, I don't always follow them!