Eaglemoon:
I deal with tendonitis myself, and it makes no difference if I'm playing the 5 or the 4 stringer as to when it'll flare. What bass do you use? If it's a passive, have you considered an active? You can turn the volume up on an active and play with a nice light touch and then string guage and count won't matter as much

I also play with wrist sweatbands (nice and tight, but not so tight to cut off circulation) if it's feeling like it's going to flare.
As for the original question, I've never retuned during a gig. I have two main basses, one set up in E and one in Eb. If I'm going to a jam, the E comes. Caveat: I play a 5, so Eb songs at a jam I just use the B string.
My first rock group is downtuned to make it easier on the singer, some of the songs that are originally in Eb she sounds best singing in Eb and there's enough of those I don't want to always be hanging out on the B string. However, one thing we do is call out notes as if we were in standard to save any brain confusion issues. I.e. if the guitarist says to me, it's in D and I play a D, it's really in C and I'm playing a C. Works well! I never switch basses in that one, if it's in standard I just go up a 1/2 step. Sometimes it would sound better if I switched, but I learned em on the Eb fingering and don't want to relearn LOL.
My other rock group plays mostly standard tuned stuff, but I bring the downtuned backup in case we want to do something tuned down. And when we do, we group them and either end the set or start the set with em so me and the guitarist aren't switching around constantly.
I did have to reset up the bass that's tuned down permanently to accomodate the B string floppyness. My original main bass fought me tooth and nail on being in Eb so it only stays in standard. My current main bass handles either tuning equally well (yea!) so it gets retuned depending on which band I'm playing with, and I switch the backups around depending on the group I'm with.
The only pita part is when I'm originally learning the songs if we're downtuning a song that's standard. What I do then is learn it on a standard tuned bass, then when I move to the downtuned the fingering is the same

. I have also used Audition to change the key of the song when I'm learning it but whew sometimes that messes with your head.
BTW, I am a woman with smaller hands and don't have any major problems with the 5-the 5's I use aren't that much wider than a 4

. I did try a 6 once and ferget it, I'll play guitar if I want six cause that fretboard is wayyyy too wide
Oh and who ever said "it's just a halfstep".... it does make a difference singing if it's on the edge of your range or it's a song you've heard 1000 times in xx tuning and its in your head that way.
