If it were me, I'd probably test just to know what we were dealing with, but it certainly would no affect that way I treated them, or their interaction with my other animals.
Then again, as most of us on here are pretty vigilant with health issues, you would probably get Tiggy checked out by the vet at the first sign of anything wrong, so would possible FIV make any difference to the ongoing care ........ probably not
Can't really advise either way hun to be honest x