Once you convert your story from book to ebook, you have two separate copies of your work. So later, if you want to make a change--even as simple as fixing a punctuation mark--you are left with the need to do it twice, or to start keeping a log of your changes. This gets even more complicated if you maintain several ebook formats: a .epub one for the iPad, a .epub one for the Nook and a .mobi one for the Kindle. It takes some experience to learn that there is a way to collapse these separate files into one, but knowing this came, for me, at the cost of having done things in an order that I would have reversed, had I known better.
So in spite of this great enthusiasm that drives you to convert your book and put it out there, so the readers can start flocking to your door, I suggest you wait until you're sure, absolutely sure, that your story is complete. Done. Finished to the last period. And then wait some more.
Once you are ready, here is a great article by Aaron Shephard called From Word to Kindle.