How to tell it's time to change metal gears?

[Gilaad]

So I have 3xHitec HS-5065MG on my 450x.

My experience with striping servo gears is all plastic geared servos, my 450x stockers that stripped when I breathed at them and my align ds520 tail servo just stripped after a very decent tail/ground contact. In each of these cases it was very very easy to tell they were stripped as they either had hard spots when moving by hand or sounded crunchy when moved by hand or under power.

I've had these metal geared HS-5065MG for over 200 flights now and probably 3 good crashes. The elevator is still totally smooth, but ailerons are not. One is very slightly crunchy at one place only, and the other has not what I would call a hard spot, but a slightly harder spot. Both still feel very smooth otherwise and run perfectly on the bench and when flying, they keep moving even when doing hard "voltag brown out" type testing on the bench and sound just like the still smooth elevator servo under power.

So obviously my question is how do I know it's time to change metal gears? Does it sound like I'm there yet? Am I risking complete servo loss and therefore crash by flying them like this? Should I change them preemptively and would you still fly them until you did?

Thank you!