I've spent my fair share of time in densely populated areas, man. After all, my family is from Chicago. I realize that you don't have humongous abandoned mine sites and things like that in your area, but I'm quite sure there are abandoned warehouses/factories here and there. Those types of places are perfect for "tweaking" your system, or giving demos for people.
And no, I don't think there's anything wrong with playing your music on "main roads". I'm basically referring to residentail neighborhoods, man. The type of places that're family oriented and have lower speed limits. If your system is obscenely loud, and you're on a heavily populated street with a 15-25 mph speed limit, your "bass" could aggrivate people for a somewhat prolonged period of time. At 50+ mph, the sound doesn't really last long enough to be all that bothersome.
The type of people that'll bitch about hearing your system for 5 to 10 seconds are just being assholes for the sake of being assholes...