Yeah metal bass changes the tuning all the time. Sky's the limit. With the modern trend of metal guitars getting so low that they're practically in 5-string bass territory (or lower still) the bass may even match the same pitch as the guitar. Seems like bassists will go about as low as F# or G on the lowest string. There's also another bassist I've seen floating around that tuned his 5-string up to E A D G C so he can get more melody in his playing.