I do this, too. I need a visual aid to remind me of voice, mannerisms and personality, or else my characters all end up the same. I don't use people I know personally because I would end up writing them, or my idea of them, instead of writing my characters (who are not them).
I don't like to use really pretty faces, unless there's a reason for it. For example, my uber-villain is supposed to be this perfect doll-like child-woman with a viciously sadistic personality. So I'm using Natalie Portman for her. I have these publicity shots of her looking just as sweet as can be. My imagination enjoys riffing off that image in horrible ways.
Otherwise, I look for people who may be beautiful but who are not perfect. People who are ordinary or normal, physically. Or people who look like the age I want, the lifestyle I want.
For my male MC, I wanted someone reasonably good-looking but kind of odd/off, kind of awkward yet interesting. Also, like my villain, I wanted an image that would contrast with behavior, rather than predict it. In MMC's case, I wanted a quiet, gentle presence that doesn't hint at but still is believable with the streak of cold-bloodedness and the physical violence. After much trial and error, I went with older Julian Sands.
My female MC gave me the most trouble of all. I couldn't find a face for her because I couldn't settle on her character. I replaced her entirely three times! Now I'm satisfied. She works with the story, the scenes flow, she seems to have a decent interplay with MMC, etc. But I needed a face so I could have her set in my brain. I wanted a woman who is physically beautiful but not glamorous, who looks "regular." Someone kind of athletic, believable in physical confrontations, but also believable being insecure and acting like a girl, i.e. younger than her age. I ended up with Jennifer Garner because there are so many paparazzi photos as well as production shots of her not glammed-up, just dressed normally, with little or no make-up on.
These are just visual aids for me, and I don't really care if the faces I choose come through in the writing, because I think the characters find their own faces in the minds of the readers. For instance, I modeled my MMC very deliberately on Julian Sands, but a friend who has read drafts insists he sees him as Liam Neeson. Totally WTF to me, but hey, as long as the character jumps off the page for him, I guess I'm on the right track.