They aren't the nicest cops in that area (they've got a rep for it).

Sorry to hear that. The 90-10% estimate I made was of course influenced by my personal experience. This might differ in your specific location, but I don't believe it will differ all that much.
I still believe that if you are nice and polite to people they'll do a lot more for you than if you're not. I also believe this is only true for people above a certain IQ. (and yes, I know this works both ways. )
Unfortunately this does not applie to most of the people I deal with on a daily basis. I'm not a traffic cop, so I rarely deal with 'the average citizen'. Mostly I deal with die-hard criminals of which most aren't too bright, hence the IQ remark.