Wife has Sumitomo's on her Grand AM with cracks between the treads. Any amount of cracking(if there are a lot of cracks) tire should be changed when possible. I haven't had the money due to work issues but its on a priority list to get done before the fall along with new shocks for her.
Almost anytime there is a crack in the groove itself, its at the heart of the tire(basically think of the deep recess as being closer to the steel belted lining, so you don't have as much rubber protection as you would if it were on top of the grooves(upper layer of the tire). Cracked rubber will flake and chip away quicker resulting in a tire wearing out faster.