First, 1:9 twist rate is a slower twist rate, 1:8 is a faster twist rate and 1:7 is the fastest (current) twist rate (I believe).
Second, a faster twist rate is desireable and even necessary to stabilize the heavier grain bullets that the 5.56/.223 are now available with.
I've read a quick and dirty breakdown that might help and it is by no means a hard and fast chart to go by.
1:9 = 40gr - 62gr
1:8 = 50gr - 70gr
1:7 = 60gr - 77gr+ (77gr bullets being the heaviest available if memory serves)
You can still shoot any 5.56/.223 out of whatever twist rate barrel you have, it just might not be the optimal twist rate for the bullet weight you are shooting and therefore can affect accuracy, especially at longer range.