In searching through the Dolby site, (a very large and extensive site), they suggest that the center channel be used for narration only, so that a person speaking on the screen will be heard from the location they are speaking from, no matter where the listener is sitting, the 4 surround channels for environment sounds, and the subwoofer channel for dedicated low end sweeteners. The subwoofer also has a crossover setting from the other channels, so there still will be sound from it even if you don't dedicate any sound to it.
I don't think there's a right or wrong answer to this. You can't apply a blanket percentage and expect it to be correct all the time.
Go with what sounds good to you. But I do think that whatever you do, it should only serve to enhance the video, not detract from it. I've heard some DVDs that have way too much music pumping constantly through the surround speakers, and it's just plain distracting.