For me it depends on what style of stuff I'm doing.
If it's Drum and Bass, it's usually the Subbass at the bottom, then the kick, then mid-bass. Although sometimes I'll have the kick go down low when there is no sub action going on.
Industrial/EBM/Gabber or Goa, it's kick. If there is bass in the same area as the kick, then it gets sidechained to the kick to duck it out of the way when the kick hits.
If it's freeform "let's see what happens if I ringmodulate a trombone sample with a low tom", then anything goes, whatever fits, and if it's fighting it gets the axe.