Hey dwtp,
I've been using generic minoxidil for about 10 months (granted it's the topical solution, not foam), and I've found it to be particularly effective on the crown, and fairly effective on the top. I didn't really see too much effect at the hairline, but that seems to be consistent with what I've read on this forum and elsewhere. This being the case, I generally just apply it to the crown and top of the head.
You should certainly try for yourself to see where it's effective for you (and keep in mind it could take a few months to notice Rogaine's effects as well), but that's been my experience with it.
Best of luck!