I have routinely ridden tires until I could see a line of bare casing down the center. As they approach that point, flats definitely become more frequent. I think of that as a tire telling me it's time for a replacement. I would assume that your knobbier will do the same but not until they get pretty nearly used up.
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My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.