I have only once taken a "spare" to a gig but it was more like an understudy - I was gigging a bass for the first time and because I had a hard time getting a suitably audible setting out of it and the amp in the practice room I was worried that a similar outcome might happen at the gig. Therefore I took a bass that was proven to work well on stage, in case difficulties arose at soundcheck.
I needn't have worried, the bass performed excellently. I played the understudy at the next gig so that it didn't feel bad
No spare basses for me in general. Taking a spare to the relatively small gigs I play makes me feel like a bit of a twat if truth be told, in the same way that wearing sunglasses indoors does. "Who does he think he is?"
I always have a toolkit, I always have batteries, I always have strings. I'll get by.
To answer the OP's question - it was set up to my liking, same standard 4 string tuning, just there to deputise in case the #1 bass wasn't cutting through.