Yes, Rogaine will help treat thinning of the hairline, and will help with hair growth in that area.
The manufacturer of Rogaine can only claim that the treatment will work on the vertex and may not on other areas because the clinical trials were done only on the crown. It will make hair grow at any place where applied topically onto the skin as long as hair follicles are present.
I myself apply Minoxidil, which is the generic name for Rogaine, in a Norwood 6 pattern all on the top of my scalp, since that's the way I'm thinning, and it works well.