A long time ago, I played bass in a few garage bands. The cool thing about playing bass was that you were in high demand. Bands that wanted to play covers would make a song list and we would learn them on our own time. Bands that wanted to play originals, they would go over the chords or riffs and I would follow them while adding my own stuff.
Really it comes down to how much time you can dedicate to practicing as a band. If its very limited, sheet music with an audio file is a great idea so time isn't wasted.