I find it funny how people react to my scanner.
Some people think i'm an undercover cop (LOL) or a firefighter (as if! i could pass off easier as a cop with the northern california 'law enforcement gut') or something to that effect, as for on my vehicle, people seem to think it's an unmarked cop car (it's a 95 ford taurus, white, with a single antenna... doesn't look like any cop car i've ever seen) and i've never been cut off since i got the antenna on there, nor have i had any pesky slow cars in front of me on the fast lane, they just move right over!
a common courtesy though- turn your scanner DOWN when going into your local grocer or department store or drugstore. the other day i came across another scanning hobbiest who had his radio BLASTING... there wasn't anything going on, but other customers were visibly annoyed.
however, the only problem i HAVE run into, was the local campus police at my college, they tend to talk S*it about people over their radio system, and i've gotten a little more trouble from then than the average student. (nothing serious, just a little "dude, why do you have that? what do you need it for?") i guess it makes them a little uneasy.
other than that, never had any problems.