I harvest from the starter just before pitching. If you are going to harvest and store the yeast from the trub of a beer, the subsequent batches will pick up some flavors from the previous batch. Washing will help somewhat. The general rule of thumb is harvest from light/delecate flavor beers and pitch to more hearty/dark beers. Hense, pitching from a blond ale yeast to a porter or stout is fine, but the reverse is not recommended.
Its up to you. In the end its more about repeatability. You don't want to produce an award winning beer you can't reproduce because you can't remember the 3 previous batches the yeast came from, do you?