I always take two basses to gigs, if there's enough room they'll both be tuned, have straps, and be on the stand next to my amp. I use one and the other is there 'just in case'. If there's not enough room, one will stay in its case, sometimes in the car, unless needed. I have never even considered changing basses from one song to another, I set the sound to suit the venue and leave it there.
I recently played with a guitarist who used 3 or 4 guitars, every some required a change, but I couldn't detect any difference between them at all!