WTB Horizon 47c - small crack in the middle of the tire
One of the go-to sites has this to say: "Cracks in the tread are harmless. Small punctures in the tire such as are typically caused by nails, tacks, thorns or glass slivers are also harmless to the tire, since the tire doesn't need to be air-tight."

I commute daily, and by the time my tires are about half way through their lifespan there are dozens of minor gashes like that in the tire.  You just want to check that there's nothing lodged in there that will work inward and puncture your tube.