As some other have said, if they aren't giving you problems, then it isn't necessary to replace them. Everyone has their own degree to which they want to ensure they won't have future problems. If having a mis-shift or a chain skip on a future ride is going to really bum you out and make you cuss every one that ever worked at the chainwheels maker, then set up a schedule to replace rings and cassette. If you replace one or the other, then replace the chain too.

I usually don't mind about problems that arise on a daily ride. I just fix after it's a problem.