I was exactly the same when i started. I played in a 4 piece metal band with some kids in my school (5 if you count the singer we never had) and mainly played the lowest string. Actually alot of metal that uses only the top few strings when it comes to bass I'm not a bass genius, I've only been playing for a little over a year but, I say do what u want. If you like to play mainly that low string then play that low string. If you want to play higher then play higher. If you want to mix it up go ahead and mix it up if you've got the experience to do so. If ur bandmates tell you "screw you, ur the bassist and ur gonna do what I say" then leave (thats actually the main reason I left that band). Also if ur a beginner than work ur way up. I'm guessing ur writing ur own music so you ought to have some freedom in what you play. Even if you decide to play lower stuff you may still want to work on stuff with the higher strings like scales or maybe learn some covers. Also even though your in a metal band try learning music outside of metal. I'm a huge metal fan and my idols go from Flea (Red Hot Chili Peppers) to Johnny Christ (Avenged Sevenfold) to John Entwhistle (The Who). So just do what you want, make it sound good with the rest of the band, but still get in some practice outside of the band with other techniques.
I did not expect to write that much and to tell you the truth thats like my 3rd post on here.
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Dean Club #22
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Bass in a Metal Band 
and the rest of my bands mates are my close friends so they support however I play. But that is comforting to hear I just don't want to end up playing at a show and like being laughed out of the place because I was making everything to simple and yeah I have learned a couple scales that I try to work into what I am playing.
