From the interviews I have read with lead game designer Todd Howard, most dragons when you see them won't attack you unless you attack them first. There are a few areas in the game, he says, where you will have no choice but to fight one but for the most part the choice is yours.
So for the most part I think the choice is yours, if you don't want to fight it don't make it mad at you...hehe
New Details On ‘Elder Scrolls 5: Skyrim’ Weapons, Magic & Dragons - Game Rant