There isn't necessarily any fixed rule, so like Mushroo said, experiment and see what works for how you want to sound. Another way to experiment besides the way he said is to set your volume wherever, but turn your EQ knobs (Low/mid/high) to zero - you should get no sound at all. Then crank the Low to 100% and play with JUST lows; pay attention to how it sounds. Then turn the lows down to 0 again and play with just your mids at 100%, and listen; and finally the highs at 100%. That will help you understand better what each frequency range sounds like and what it's contributing to the overall sound.
The lows really give you your "fundamental" tone, the bass-ness of the bass. That's where the "warmth" and "roundness" is that they were talking about. But it tends to get muddy and indistinct with just that. The mids give you a lot more note definition and articulation, and a certain "bark" in the tone. Crank the mids way up, though, and you'll start to have a "honking" sound. The highs also help some with note definition, and give a certain "sizzle" or "growl." Effects pedals, I find, like having some highs to work with. But that's also where a lot of noise is, the squeak of your fingers on the strings and the clank as they hit the frets.
In general, if you're playing something that's more jazz or motown in sound, dial your highs and mids way back. You want smooth and warm for that.
For me, the default rock setting is to bump up my lows and mids to maybe 2 o'clock (like 7/10), and roll back the high to maybe 10-11 o'clock (like 4/10).
For a very "modern" sound, especially playing slap bass on funk tunes, crank up the lows and highs and dial the mids back (that's called "scooping" the mids).
Like Mushroo said, be prepared to LISTEN to anyone else your playing with and adjust to fit what sounds good in the group. A great bedroom tone is not always a good band tone. At one of my first real gigs, I had this driven home to me. I was in a band with a more experienced guitarist, and at practice he asked if he could change my settings. I said sure, we fiddled around, and basically he scooped my mids and said that was the tone he liked - it was like 9-3-9 or something. Next gig, first song, he shouts across the stage at me to turn down, my sound was way too "boomy." When he said "boomy," instead of cutting my volume, I changed my EQ back to the 7-7-4 I had been using before he changed it. "How's that?" "Great, better, thanks!" The scooped setting sounded great at home, and might have sounded great in a different mix of instruments, but was not right for the way HE had HIS settings.
