While the auto-preview function in SE is annoying when I try to write a very long answer/question, no preview at all isn't convenient either. In fact I would say "no preview" creats a technical barrier to especially new users, for new users(talking from my own experience) would like to test by trial and error and learn how to compose a nice-looking post. For example I was just trying to figure out if double dollars $$ would create a centered math formula, but it seemed to be impossible without actually posting a question.
Could we have a manual "preview post" button? I remember www.physicsforums.com has this function, which helps a lot.
Aside: By the way let me just test double dollar here, and edit away if necessary,