Which Rs are rolled in Italian?

Is there a rule about which Rs are rolled in Italian? Somewhere I read you don't roll an R that is between two vowels. In any event, when I listen to recordings of native speakers, it doesn't sound like they roll all their Rs the same way. Certainly, it is easier to pronounce some Rs (e.g. ones immediately following a "d" or a "t.") Perhaps this explains why some Rs seem to be rolled and others (e.g. ones between two vowels) are not?