It basically depends on how stretched the chain is, doesn't it Tony? I just put a ruler along the chain and if 12" of links only measures a fraction over 12", then I reckon you're not going to damage the rings. On the other hand, if it measures 12 1/8", then the chain needed to be replaced in any case. It's if it measures 12 1/16" that you've got a dilemma - it'll work fine, but it'll wear out the rings a bit quicker than you might want.
I think it's a myth that you need to replace the cassette every time you replace the chain. If you don't let your chains get to 12 1/8, I reckon a cassette can outlive a few chains.