I set up this board specifically for double duty:
While I do more bass gigs than guitar, I do have a couple of projects (solo acoustic and a power trio funky rock thing) where I play 6 strings. The DD-2 I've had for ages, got it for guitar, and it works fine for bass. The Bass Drive I bought back in '02 specifically because while it is voiced for bass a lot of guys like it for guitar as well. For guitar I also use a very old Vox wah and had a Voodoo Labs chorus. I haven't tried the wah for bass but am not really interested in going there, instead preferring the envelope filters. I sold the Voodoo Labs chorus and bought the Analogman specifically because it *kills* on both bass and guitar due to the deep toggle switch.
The envelope filters are new toys that I bought for bass because of a straight funk/acid jazz project but last time I did an informal guitar trio gig I found I liked the auto wah a lot. Both the Maxon and the Menatone work equally well on guitar and bass - you just need to tweak the controls a bit.
Sooo....as people said above, no hard and fast rules, just try things and see what your ear likes.