I can't speak for all universities, but where I go
, there is no prerequisite for taking music theory courses, so long as you start at the elementary level (they offer: elementary and intermediate theory classes, with each section taking up a year's--two semesters'--worth of classes).
I'm an English major, but I took the year of elementary theory when I was a Sophomore, and I'm currently involved in a composition/arranging class as a Junior with 3 semesters to go before graduation.
Next semester, I will probably start intermediate theory and take another form/analysis type class.
I take these classes simply because I like to learn as much as possible about how music works. While it's true that music theory can be learned at one's own pace, I find it easiest to learn in a classroom setting, where the trial and error of putting theory into practice is made easier by having classmates and teachers to help you out. Other people may experience different results, so I suggest just finding out what works best for you.
That said, when I come home, plug in the (bass) guitar and start playing, I don't worry about theory. Guitar playing, especially guitar playing of the metal variety, is not something that tends to require knowledge of theory (or high levels of musicianship, for that matter).
Look at a band like Gorguts, as an example:
One of the band's main songwriters (Luc Lemay) has a bachelor's degree in music composition, but still admits that he writes metal by ear, as he did in his early days with the band. I believe he said something similar to what others have said above me:
Just because you know theory doesn't mean that you have apply it to everything you write; if anything, it's more practical to use it within the contexts of "band" music as a language that one can use to communicate with other musicians, and to discover, if necessary, how/why certain sounds function the way they do, at the scientific level.
When one starts writing music at a level of complexity that far-exceeds the reach of metal, where there are more than two or three active voices, each with its own unique range/timbre, that is when theory is most helpful as a compositional tool.
In simpler musical contexts, (like metal) it is much easier for the composer to get by without knowing the science of how the various elements in his music are reacting to each other; the more elements that get added, though, the closer the composer has to scrutinize the reactions, otherwise, the music is likely to combust.