the rings are good. depending on which ones you'v got some come with little pours some are smooth. the smooth ones filter out all the partials in the water. there maniacal filtration the ones with pours do the same but also provide a surface area for bacteria.
it all depends if you need more bio filtration if your tank cant cope with the ammonia produced then replace them to get more bacteria in there to keep it down. although if it doesn't need anymore keep them because it keeps the water looking cleaner
hope this helps