For me, it just depends on the gig.
Last year, I filled in for my Dad in a Stanley Bros. tribute band. Any bass guitar of any kind, would have been completely out of context there. The Stanleys were about as primal and raw as it got, and no, anything other than an upright would not cut it, visually or tonally. I'll take it a step further; to really nail the Jack Cooke tone, I played the Ol' Man's bass, strung with synthetic gut strings rather than mine and it's hybrid chromesteel wound on perlon. Way too much *boing* for traditional bluegrass.
With the neo-oldtyme band, I took even more liberties. Most of the music we were playing in that band was written without a bass part in mind, or even rhythm guitar. It's fiddle/banjo driven, so the rhythm section was wide open to interpretation for Brian and I. Probably very few of you ever heard it, or will, (just a couple folks I burned CD's for) but we put this moving, arpeggiated, sometimes melodic/harmonic line underneath the lead instruments. I played a fretless Turner Renaissance 5-string bass on 90% of that project, something the traditionalist element in oldtyme music circles would not accept under any circumstances. But we got booked easily as an Appalachian String Quartet. And the folks paying us couldn't have cared less. I played my Alembic Custom fretless 5 string, right up to the very last gig we did. I even played my fretless Hyak on a couple. No disrespect to the oldtyme crowd, I can hang with any of them on my doghouse, any day. But I play what I want to on my gigs.
With the Harwell-Grice Band... those guys wouldn't care if I played a washtub so long as I held their pocket. They think my Alembics are cool mainly because of the Grateful Dead connection. The music though, is perfectly suited for an electric bass guitar. It's a loud, plugged-in jamgrass band. You would have a very hard time getting an upright to cut through that racket. And honestly, it's just too damn physical. For me anyway. Those guys play hard and fast... Josh is like the Tasmanian Devil on Red Bull. If I ever do get back to gigging with them, it'll be on bass guitar.