Distortion is pretty much an absolute necessity in heavy metal music, rhythm playing or any playing for that matter. I don't really think that overdrive works that well in true heavy metal. I think that back in bands like Black Sabbath in the '70's and when metal was really starting to take off, I mean they were using Marshall amps cranked and it's a very saturated, compressed sound. Nowadays you can get that in a pedal.
I feel like overdrive is too transparent sounding. It's too clean and it really doesn't achieve; when you're playing rhythm guitar and heavy metal; it's just not saturated enough. So the differences there are that in rock music you're seeing the spectrum of using an overdrive pedal to get really the sound of a cranked '50's tweed Fender amp versus a Marshall Full Stack at high gain, which is used in bands, you know anywhere from Black Sabbath to Slayer Metallica.
When you're soloing you can use a boost. A lot of these amps have two channels; a clean channel and a dirty channel, some even have three different levels of distortion. I've seen a lot of amps that have three channels that are one clean, channel overdrive, channel three full-blown distortion. Those are really good for heavy metal, because you would use channel three for example, switch to channel two for a verse or to clean it up just a little bit, but not too clean or use it as a boost.
A lot of amps also have a separate boost master volume setting, so that you can boost for solos. As far as pedals are concerned, you could have this Metal Zone pedal for all your rhythm stuff, and then you could have another pedal hooked up to really bring out the level of the solos. But for the most part for heavy metal rhythm playing you really need a good distortion pedal that can really saturate the tone, so that even at a low volume you're really hearing all of the frequencies versus clean, which in metal just doesn't cut it at all. So that is using a distortion pedal in heavy metal music.