Skimmer here. I don't skim every time, and I don't particularly care, but some beers (I haven't determined a specific difference) have a lot of foam, and if there is too much, it makes it more easy for a boil over. If it looks like there is a lot of foam, and I am worried about a boil over, I skim until I get a bit of a break in the foam for all the steam to escape. Either way, I haven't noticed much of a difference at any step along the way.