Nope, I don't use a screen protector on my Original Series Black Stainless Steel Apple Watch. The Sapphire Screen is holding up well. No scratches or micro swirles to speak of.
It's you get the stainless steel model, you won't need a screen protector with the sapphire glass. If you have a sport model, yes it will scratch. I have a SS so no I don't use one.
Have never used one. Series 2. I can tell you that my Series 1 Aluminum did get a nasty scratch on the face even though I babied it. I've had the newer one since released and wear it everywhere and work out with it daily and it doesn't have a scratch on it.
I did have one, but removed it when it started peeling off. And now I have a couple of scratches on the bottom two corners... (not all that concerned about it, though.)
I bought one but it kept coming loose in the corners. Decided it wasn't worth it. Hopefully I can be careful and not scratch the screen. I have the aluminum model, not SS.