I've never yet had to change a bearing (all castor ) but I like to check every so often that they feel nice and smooth just in case. With rings I'd only change them when the engine gets harder to hand start. I've only done this once with an Enya 45 that had a couple of hundred hours running and was no longer starting with one flick. Once running it was as good as ever but because it was used for competitions it was necessary to have fast starts. I've never owned a 4 stroke though so I'm guessing it's never hand started which makes that method of recognising worn rings impossible.