It's amazing how much dirt can get into your wheels in a short amount of time. I was out in some sand one time and in less the five minutes I had about 2 tablespoons of sand in my tires. Dirt isn't quite as bad as sand but it can get in there pretty quickly too. I like this idea I'm going to start putting holes in my tire right away!
I think you'll find that a pin hole isn't big enough. The rubber tries to seal itself off. A half inch slit work's best for me. I just uses scissors to cut the small slit, and I've never had any trouble with them tearing.