Most of the time a non slipping diff is correct, as posted above. Sometimes, if the track is garbage, you have to let the diff slip. There were videos of a Tamiya nats where one of the top qualifiers had his diff just screeching from slip all the way around the track. I was not there, but I heard the track did not have much grip that weekend.
One of the first Tamiya nationals I went to one of the guys had his pit man on the track adjusting the diff during hot laps. He did have the old F103 diff on there. Same thing, we had to run the stock F104 rears, which were pretty low grip.