I've got a van as a daily and drive around with the back up 90% of the time with no issues.
I got pulled over for an RBT a few weeks back and the guy made the comment that it must be pretty good having the breeze going through, so zero issue there.
I know of a member here who got pulled over and instructed to shut it immediatly as it was a saftey issue.

I think at the end of the day it's down to the cop being a person that's going to have good and bad days. Plus how you talk to them always makes a massive difference.