I use the same razor for head and face. Never had any trouble that way other than blades getting dull faster. If it's the Mach 3, I get about two good shaves of head and face before it goes a bit ratty. Will sometimes be able to stretch it to four but that isn't so comfortable. With the double edge razor, I use blades once on each side for head and face. I'm still trying to decide which ones I like best then I'll buy a bulk pack. Currently spending about $5 for a pack of five. Cheaper than the Mach3's still.
Yeah, I know I'm getting ripped off on price there. Just got to decide which ones I want to buy 100 of. Where do you buy them from? I'm a little hesitant to use ebay etc (security paranoid).