I think it's a good idea keeping the volume pretty even, and then the players can adjust as needed. For example, it doesn't take much effort to go from soft fingering that's barely noticable to raging slapfunk madness on a bass.

A general rule of thumb I use (which may or may not work for you) is the bass should be loud enough in relation to everything to be heard clearly but not loud enough to drown anything out. It also depends on the skill of the bass player and the guitarist, and it depends on what style of music you're playing, and it depends on the guitar's tone. For example, if you like playing with your upper mids cranked it might be a good idea to give the bass a little boost to balance it.