Yeast washing as often described around here is actually yeast rinsing (washing involves acid) and is quite controversial. Evidence shows that there is just as much viable yeast, sometimes more, in the discarded "trub" and liquid layers as in the white layer. You're also displacing yeast from its happy home in the perfectly pH balanced and sanitary beer it created for itself, into a potentially hostile and infectious environment. For these reasons many of us choose to simply save raw yeast cakes, trub and all, under a little of the beer it was created in. It's easier, at least as reliable, and involves less effort.