My friend took his tire in to a shop to get patched and the guy there told my friend that he needed new tires soon. One of the reasons the guy gave was that the rubber in the grooves was cracking. I saw the cracking and it was very miniscule. Granted the tire is pretty old, probably 3 years old, but still has plenty of tread on it.
In short, what does it matter if the rubber in between the treads is barely cracked, every so slightly? My friend drives his Tacoma like a 70 year old woman, ever approaching anything near 85 mph, FYI.