What is weird, is once the game decides it is going to spawn one, it is going to spawn one, which is why I can keep them from the towns this way.
Cheers, I'll keep that one in mind!
Also, by what you have found, have you managed to work out what kind of time period, may be between dragons spawning, or how many fast travels before one spawns, for example, you travel to solitude, a dragon spawns, 7 fast travels later, you go to markarth, another spawns, then the same pattern, every seven?
Thinking of this, I'll try it now.