I try to choose someone who looks like what I picture in my head, but is also less recognizable. Usually I use photos from modeling agency sites or no-name people that I've seen in ads or music videos. In the original banner for Broken, the girl I used for Claire was a model I saw in a Sketchers ad in a magazine who had the look I wanted for Claire. When I wanted to write a sequel and make more banners and couldn't find more pictures of the same girl, I made new ones using Bryce Dallas Howard, who I'd seen in The Village - she wasn't as well-known then. I never pictured Claire as Bryce Dallas Howard when I wrote her, though.
The only other time I've used a celebrity for one of my own stories is Curtain Call - I used Zooey Deschanel for Cary on the banner that's on my site because I did base Cary's vintage look/style on her. If it was a movie, I would have wanted to cast Zooey Deschanel, so it was okay if my readers pictured her when they read the story.
Otherwise, I try not to use recognizable faces for exactly the reason you mentioned, Mare - I don't want readers to think the story is about Nick and Cameron Diaz, or whoever. I guess it's the same as casting directors who want to cast unknown actors for big roles because they don't want viewers to associate that face with some other role.
I don't know how many readers actually picture the face on the banner when they read, though. There are very few stories on my favorites list with a random female in the banner, and I can't say that for any of them, I actually pictured that face in my head as I read. I think we make up our own image of any character based on the description that's given, or lack thereof, more than what's on the cover/banner.
(Btw, as I was writing this, that hedgehog commercial came on, Mare - I hadn't seen it yet, but you were not just making it up. Cute!! Too bad it wasn't an anti-depressant commercial; that's the only thing that could make it better LOL.)