It really depends a lot on the tone of the guitar and bass in question . . . as well as the style of music and so on. They all sound at least a little different, and thus require different strategies on this.
Very generally speaking, there are some general rules I'll follow. Here's an example:
Most electric guitars naturally have a scoop in the midrange, right around 1000 hz to 14000 hz (which is, subsequently, a good range to boost on the bass guitar if you need more definition out of it . . . there's a natural pocket there for it). They'll also have a higher-midrange peak somewhere in the neighborhood of 2000 hz . . . depending on it's unique tone, it could be anywhere between 17000 to 26000, actually, but it's usually somewhere in that neighborhood.
They'll also tend to have a couple more peaks . . . one between 300 - 400 hz, as well as a bass peak somewhere in the 70 to 150 hz range. This is the range you should be concerned with, because that's where the bass guitar likes to hang out.
Ideally, you want the 6-string to have some action in that range. If you just cut it for the sake of giving the guitar some breathing room, it will wind up sounding thin and wimpy. Ideally, you want to try and dial in a guitar tone during the tracking phase that has some action in this range, yet doesn't conflict with the bass, which is tricky.
Let's say the bass is peaking out mostly around 100 hz, but also has some action going on around 159 hz. Now let's assume the electric guitar has it's bass peaks around 159 hz, also. In this instance, you might want to try cutting a little 159 on the bass, or better yet, multiband compress it at 159 hz.
Listen to it, and if it sounds okay that way, then go with it. You've just opened up some breathing room for the guitar without sacrificing the bass sound or having to make the guitar sound wimpy. That's a best-case scenario.
Worst-case scenario might have both the bass and guitar fighting for the exact same frequencies at 100, 159, etc.
This should be a sure sign that you did a bad job tracking.

Really, you should be listening for this stuff while you track. If it sounds like there's a lot of mud, or you can't distinguish the bass from the guitar while tracking, then you need to experiment . . . try a different pickup, change the mic position, fiddle with the amp tone . . . whatever you have to do. Otherwise, you force yourself in to unnecessarily difficult situations come mixdown time.