I agree with the "if the tire isn't swelling in the area of the hole, it is probably OK." I also put a bit of Shoe Goo in the outside of the hole to keep anything from getting in there from the road (I do that after rides, not during, and for slits that didn't result in flats, too) Looking at the holes/slits is a good way to tell how worn the tire is and these days I err on replacing early vs. fixing flats on the side of the road.