Whenever faced with this question I go for comfort reading to 'Torrens' of The Motor Cycle', whose long-ago book "The Motor Cyclist's Workshop" has a great chapter entitled "Is It Worn Out".
He says in the Seventh Edition, published in 1961, that:
"The term "worn out" is a poor one. The gap between the ends of a ring, contrary to widely-held belief, is not the deciding factor as to whether a ring should be scrapped or not. Much more gas leaks round the back of a piston ring than ever passes through the gap.
Small gaps are a fetish with some people . . . the only time they really matter is when they are too small and then there is the devil to pay. . . . err on the side of large gaps . . .whether to scrap depends on whether there signs of gas leakage".
He goes on to quote the "service department" of "one of our most knowledgeable manufacturers" . . . whose practice, "in the case of an 82mm bore is to replace when the original gap of 15 thou has increased by 30 thou, making 45 thou in all".
I don't know really, but I certainly don't replace rings just because they have larger gaps than when new. If their springiness has dwindled a lot compared to a new or known good one, then I'll replace them, or if there are marks of blow-by, or if there is a step on the top one from whacking a carbon ridge, or if there is smoke or an obvious lack of compression not due to valve trouble. Otherwise, I tend to leave well alone.
When Torrens was writing, people were using their machines as ride to work and family transport, on tight budgets, so he was probably trying to discourage people from spending money they hadn't got on parts they didn't really need. Advice I quite like, being mean, and lazy as well when it comes to not looking for problems to solve when I could be riding the thing.
It's a good book, and Torrens was widely respected in the day. (Shame one of only two commercial advertisements in the rear of the tome shouts - 'The man who knows motorcycles . . . chooses Triumph'. Hmm! I'll happily remain ignorant then.)