Should our sink waste hole reducer be sealed?

Our steel kitchen sink has a circular reducer which takes the diameter of the waste hole down from something like 3.5" (or whatever it is in mm) to the usual size. Presumably it allows you to put a waste disposal unit in there, or something. Since the sink was becoming a bit smelly, I had the opportunity to dismantle the pipework underneath, and scrape away a lot of lovely slime, much of this under the reducer. It struck me that since it is not sealed, a fair deal of water is going to trickle under it, and the slime becomes inevitable. The trap underneath has a large bowl at this point, so it doesn't drip anywhere else. But it seems to me that it would be better if it was sealed. OTOH, I have to assume that it was originally fitted by a pro.