Just a thought:
It is VERY rare that I will go to a "call" to see the action, one exception is in the case of plane that made an emergency landing on route 37 last month. Upon my arrival I saw quite a few people with scanners in their hands and on their hips. Perhaps it would be smart to leave the scanner in the car. If you absolutely have to monitor what is happening then stay in your car. I say this because the chief of police was on scene and he definitley noticed all of the scanners in the crowd.
It is not morally wrong to carry your scanner with you, BUT when the chief, capitan, or Lt's see scanners, it may possibly set off a lightbulb in their heads saying, "lets go to full encryption". Not so much in the case of a plane crash or any incident like that, but if they see you at the scene of something where information is of a sensitive nature, or is to be kept private (homicides, suicides, shootings, etc.)
I'm sure I will get replies saying, its our right to listen and all that, which I totally agree with. But think of the concequences, if scanners are spotted in the hands of the crowd during major incidents it definitley rubs the police the wrong way, especially brass.
I think the no. 1 rule in scanning is dont get in the way of police, fire, ems, etc.
The no. 2 rule in scanning should be dont get in the way... while holding a scanner.
Getting in the way can and very often does include on-lookers, as the police now also have to worry about their safety. Try and keep as much distance as you possibly can from the scene if you absolutely have to go and see.