I briefly started what I believe to be the MQ, which involves Bleak Falls, but then stopped because I wanted to just venture around. So your saying if you
don't play the Main Quest you won't see any dragons? Thanks
That's correct, dudeman. The whole 'dragons reappearing' phenomenon is directly associated with the main quest storyline and without it, they are actually not really relevant to the rest of the game world.
As the other posters said, once you complete the early quest to help defend Whiterun from the attacking dragon, you will start to experience random dragon encounters, and the higher your level, the more you'll have (so low leveled characters will not have many at first).
Stannie