The answer to the primary question is, 'yes.'
Otherwise, a plug that is 'damaged' due to being in a too rich motor isn't ever going to 'clean up' to the point a clean plug or plug that was in a 'clean' motor ever would.
So, no..those wet, black plugs won't clean themselves up. You would be more likely to just end up baking the crud on!
Once you carry a few of those used plugs around for awhile, I'll bet they are less than spotless anyway. Those plug caddies are good, but pretty dang big, too!
Certainly, once a plug has fouled..toss it. It won't be reliable.
Guess it depends on how much of that (reliability) you're after.
imo and all that junk
It's good to see a number of youse guys actually going to the trouble of sorting your bike's jetting requirements out. You'll be real happy with the result!