Personally, I only like to rebuild my diff if it starts to feel gritty. If the diff is still operating smoothly, there's really no reason to rebuild. It's still doing its job.

If a diff gets too gritty, however, it stops being effective and you'll notice the car won't handle very consistantly. And you can feel a gritty diff very easily just by turning one wheel. It should turn smoothly and the opposite wheel should turn the opposite direction. If you feel any grittiness, it's time for a rebuild. Or, at the very least, a good cleaning and regreasing.