As with tires on the car, I also try to gauge the tires' ability to remain planted on less than ideal surfaces. When minor amounts of gunge and water noticeably reduce the grip, then for me that's the time to replace them ... no matter what other indicators might not exist at that point. For me, it's just not worth failing to have that "normal" grip. On a car, that's one thing, since at least you're somewhat protected in the "cage." But on a bike, it can easily mean a face plant in a ditch or riding into oncoming traffic.