My clippers are human ones and I've never used a guard... I actually tried using a gaurd once but the blade clogged nearly immediately so I had to take it off.
Post-guard-removal, I discovered that they actually work just fine without a guard. I think mine have a "size 10" blade on them which, if I remember correctly, is the size you want to make sure you don't go all the way down to skin.
I might be wrong, but I think I remember reading that 15's are for whiskers/muzzles (closer clip) and 10's are for body clipping/roaching. You could probably use a 15 for roaching but that might be getting awfully close, I've used a 15 for roaching before and I definitely prefer using a 10.
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.