Nicks & Cuts in tires

I'm riding on a set of Michelin Axial Pro tires that I just love, not only
because they handle well but because they seem to resist flats. Over the
past few years they have been cut or nicked in a bunch of places along the
tire surface, but it hasn't affected handling or their ability to hold air.
Periodically I fill in these cuts or nicks with Goop, which road wear evens
out nicely as I continue riding on them.

My question is this: How do I know when it is time to replace these tires
or when it is no longer safe to ride on them? I ride on the road and
highway shoulders, where there is a lot of junk that can cause flats, but
these tires have proven themselves better than anything I've ever used
(including the kevlar lined tires which seemed to flat at least twice a
week). I've only had one or two flats in the last three years of riding on
them...

Any suggestions/information would be appreciated.

Chuck