This is by no means the absolute answer, but on a touring car I'll set the diff tight enough that it will never slip under power, and the limit of tightness for me is that when I turn one wheel the other wheel turns in the opposite direction rather than the same direction (with motor and pinion in place.) in other words, if I can make the drive train turn the motor by turning one wheel the diff is tighter than I'd run it. This is with a brushed motor in place... some brushless are so "cog" free that even a fairly free diff wil turn them over.
On a 1/12 pan car, I always run the diff as free as I possibly can without letting it slip under power. I don't ever use a tighter diff as a tuning option (although I know more than a few racers who I repsect that DO run their diffs a bit tighter under certain conditions.)