Well, I will tell you this. I started using minoxidil when my crown started thinning and my density started to go along with my temples. Almost 5 years later after applying minoxidil all over my scalp (along with Propecia and Nizoral in the mix) my crown has filled in and I am a NW2 at the age of 38. Treatments work different for everyone but catching things early (as you did) helps alot because it is much easier to maintain than to regrow. So I would say use it all over if you can afford it.
Good Luck