There are so many things that it depends on....like what I'm listening to, who is in the car with me, where I am, what time it is, all kinds of stuff....
Most of the time I'll turn it down in parking lots and at long lights, but if I'm with friends and being stupid there are times when I purposely want to be a jackass. If somebody next to me is talking on their phone or has their window down, usually I'll turn it down out of courtesy. If somebody pulls up next to me blasting some rap music, I usually put on the Captain Planet theme song and blast the crap out of it just because it's funny. If I'm angry I tend to listen to my music louder and not give a crap about anybody, but if I'm in a good mood I listen to mellow music and don't turn it up all that loud to begin with. Basically, it depends.