Yes they do practice.
My buddy grew up in Oakland and would hear gun shots every night.
They weren't shooting each other everynight they were playing with their guns.
This seems like a pretty good indicator that some of them do indeed practice. I have heard this other places, and I agree they aren't shooting at each other every night.
I still think there are a lot who don't practice, judging by the results of some of these ridiculous gunfights. But then again, I have seen a few articles about cops who had their gun jam in a firefight (bad technique or weapon maintenance.)
It seems like a lot of people in this thread are just repeating stereotypes. Thanks to those who posted relevant info.