I've long been in the habit of using sunscreen. It's particularly important on snow, because the sunlight gets reflected back up and into your face on places that you would think would be covered or shaded by a hat. btw I like those dorky hats with capes.
I've read that recent surgical scars are particularly vulnerable to skin cancer, and I have them in the front and back of my neck, so those areas get slathered.
I never apply sunscreen above my eyes, because otherwise when I sweat it runs and burns. At higher altitudes the sun is more intense, and you will want to reapply the stuff a few times during the day (Shasta for example).
I personally like the neutrogena sport lotion, spf 55 (I guess now it comes in spf 70 too). It isn't so oily. I also picked up a sunscreen "stick" from REI last year which is kind of convenient.