You dont want them so tight that they bind but they shouldnt move too much sided to side. With the diff out try turning it by hand. When you are turning it pull and push on the diff cups and see if you feel any binding. If you do you need to shim them more. The problem is that when you are making turns the dogbones put pressure on the cups which causes the spider gears to bind and break if there is too much play.
One of the biggest things to avoid so you dont blow your diffs is 180's on power. That just chews them right up