I have had several rock chips on my windshield, but have had them fixed to keep from spreading.
I haven't seen anyone mention this, but if you have full coverage insurance 95% of the insurance companies will pay free of charge with no deductible to have your rock chips repaired up to 3 times a year (if I remember correctly). It doesn't increase your rates, it's just one of those free service they have. As an undergrad I worked at a carwash and use to fix glass chips there.
I was driving down the street about a week ago, had a rock thrown into my windshield, drove straight to the nearest glass shop and had them fix it through my insurance. Saved 50$ and helluva lot cheaper than having the chip spread into a crack.