Well, yeah. I don't either. But worrying that somebody else playing your bass on one or two songs is going to make your strings go dead is downright superstitious.
How about this rule, to put the matter to rest... If I'm comfortable with you shaking my hand, then I'm comfortable with you touching my bass... Does that sound fair?[/QUOTE
Early on when I was playing guitar somewhere, a friend of mine showed up. Asked him to play, and let him use one of my spare guitars, not a big deal, oh and the band was fine as he was an additive to the band that night as opposed to replacing anyone. My rig was split between two ampsso no issue.
A set later, grabbed the guitar and and though he had wiped it down, it clearly did not feel right. Took some serious cleaning to get it back to where it needed to be. Clearly my friend is one of those people who's body chemistry is quite different. Everywhere his hands were was a grunge that was left after the perspiration dried.
Regarding instruments in general, not sure about you but some people make their living off of their instruments and they are tools of their trade. Nothing wrong with people not loaning out the "tools of their trade" to perfect strangers.
Try asking that of a carpenter or mechanic sometime. Let me know how that turns out.