Rear diff is very important to keep smooth as if it is too tight or notchy you will get some pretty inconsistent handling as you put the power down out of corners. It should rotate smoothly but not spin freely.

Ideally you want to combine this smoothness with a diff that doesn't slip... not always that easy to acheive and depends on the components you are using, for example I find that diff halves made of a harder material (alloy, steel) are easier to keep smooth without slipping. Same should apply with harder balls, they will stay rounder and thus smoother under greater pressure. Shame that nobody has started to make hardened diff rings yet...

Also don't forget the importance of the thrust bearing, this should be well packed with grease to run as smoothly as possible, it has to handle loads similar to the main balls.

If diff is still rough after being thoroughly cleaned, visually checked and rebuilt, then it is time for new internals.

Front diff is not so critical, if it is a bit rough you will be less likely to notice any handling problems, but it should be built to the same standards as the rear and will normally be set a little tighter, this adds a little stability on power.