this is what I pretty much do now too, if i'm making pretty much the same beer I just leave the yeast cake in the carboy and rack the new batch on top. I used to keep a yeast bank in the freezer and step it up to what I needed. this is much easier to do and takes less time.I only use 2-3.
My progression is usually dry/liquid yeast with starter for a relatively mild batch, say a batch of Centennial Blonde or a simple Pale Ale. I then simply collect the yeast cake in 4 Mason jars (no real washing) and store them in the fridge. I'll then pitch each one into another 5 gallon batch, usually an IPA or double IPA. After that, I figure the yeast is probably pretty well done (high gravity worts stress the yeast more) and "retire" them down the drain.