Sewer clean outs are really handy whenever you have a sewer main clog or when you have a tough stoppage to clear. One thing to keep in mind is that clean out caps can leak just like any other plumbing fitting. It's a good idea to know where all of the clean outs are located in your home so you can check them periodically.
While crawling under a house recently I came across three clean outs and all three caps were leaking.
Even though the leaks were minor, it isn't something you want to ignore (this is a sewer line after all). Since the clean outs are located in the crawl space under the house it isn't a problem you are likely to notice. One way to catch the problem early is to make it part of your routine plumbing maintenance to check the clean outs.
All it took to stop the clean out cap leaks was a bit of plumber's tape applied to the threads and the right amount of tightening.