I'd say nothing is a better tester than your nose and tastebuds.

Pluck a leaf. Rub it between your fingers to release the oil. If you get that wonderful pineapple mint smell, then taste it. If it's OK....don't worry about it. Let the plant do it's thing.

JMHO.

Yes... that happens with the pineapple mint, I have noticed... also happens with an elderberry I have. :D Some plants do revert and you have to prune those...but unless the mint starts making a patch of the reverted, I wouldnt worry. Also, some plants, and this irritates me, I never know which, depend on the amount of sun to be variegated... some need more, some need less. I am losing the variegation on a philadelphus because I can't give it enough sun. :( Sorry, I think I am rambling.

Almost all variegated plants will eventually revert to the original green. You really need to remove all the green leaves as they appear. I suggest taking a few cuttings - some will be variegated, some not, but it's worth a try to retain the variegation. Still, the flavour won't alter if you just let it do what it obviously wants to do.