I believe Marduk and Dimmu Borgir only use Standard Tuning.
Darkthrone now seem to tune the whole guitar down a whole-step.
I've actually just recently tried C G D G B E. It's kind of cool - extends the lower range of the guitar, but keeps the top four strings in standard. Much heavier sounding, and allows you to play 5th chords on the 6th/5th strings, AND on the 5th/4th/3rd strings.
there is no right and wrong way to do it - sometimes you need a creative boost, and changing tunings can help.
But I also like to try to play along with metal bands, and find it difficult because it does seem like every other band is using some weird down-tuned setup.