It depends, really. . . .
In some campaigns - especially those placed in a specific published campaign world - I'll use whatever the campaign setting uses. In my homebrew I created my own pattern for dragons.
I've played under one dm whos stated that all dragons were brown, and unless we had ranks in Knowledge (dragon) we could not tell (from horns, ridges, etc) what specific breed it was - and therefore its alignment, breath weapon (until it was used), etc.
I've also played under another dm that always did them by the book, more or less, but occasionally granted a dragon a personality that didn't really seem to fit its alignment. Of course, this was also the dm that had a true neutral dragon quasi-deity whose main purpose was to hunt down and eat the great wyrm dragons - keeping the population down, especially those that stood out too much.