a complete rebuild isn't necessary. But make a habit of re-building the diff when it feels gritty. I'd say, either judging by dirt in the gearbox or every 6?-ish diff rebuilds...you might want to flush out the whole tranny.
I'm wierd, so I do it pretty much everytime I re-build the diff...I just figure that while I have it all torn apart, I maiswell just clean everything while I'm there.
TQ Racing Wire