I did Bleak Falls Barrow and killed the dragon at the Western Watchtower in Whiterun fairly early (up to level 10ish), and then decided to ignore the main quest until I'm a god-like ninja-archer-tank thingy of awesomeness.
That's mainly because I'm a scaredy-cat and didn't much enjoy being handed my ass repeatedly by flying scaly things... running away being the better part of valour and all that.

Also, I don't take orders particularly well and my attitude is "Stuff 'em - if those dudes in High Hrothgar want me so badly, let 'em send a cart for me. Damned if I'm climbing 7000 steps to be lectured about duty".
So since then I've been wandering around, picking flowers, dungeon-diving, pimping out my crib in Whiterun, smithing, side-questing, brewing noxious concoctions, ogling Lydia, etc etc.
Basically procrastinating my little heart out, and I'm level 26 at this stage.
I do NOT have the main quest active, usually I just activate whatever 'fetch me 10 rat tails' miscellaneous job or proper side-quest I'm currently in the mood for.
Here's the thing: I get random dragons fairly frequently, one or two every gaming session of about three hours or so.
They frequently happen just after I fast travel, and seem to have a fondness for terrorising the poor people of Riverwood. I've had three random dragons there.
So it looks like everyone's mileage does indeed vary.