I don't use a screen protector on my Urbane, but I do on my Moto360. Why the difference? The Moto360 has a flush face making it particularly vulnerable in my eyes, while the Urbane can lay face down and the glass surface won't touch anything due to the raised bezel. Now, if I frequently had the misfortune of bumping my watch face into wall corners and such, or a job that put my watch face in reasonable danger, I'd definitely get a protector. The screen protector on my 360 works fantastically, and you never know it's there.
Definitely a personal decision, and no wrong one.