It's quite difficult to get the formatting right for Smashwords; chances are there will be at least one mistake the first time you publish a new work. But that's not good news. The problem is, as soon as your book is published, it becomes available on the Smashwords front page. This is your moment of glory; your fifteen minutes of fame. And you don't want it to be spoiled by a sloppily formatted book. You want your readers to get the slickest, fanciest version of your book - after all, you want them coming back for more.
I asked Smashwords tech support about this. I got a very nice reply from Aaron, but essentially there is no way round this. It is something they are considering implementing, but not right now.
This is where my tip comes in. I have a book in my dashboard called 'Do Not Buy This Book'. To encourage people not to buy, it is conveniently priced at $100. (So, er, if you do buy it, thanks very much).
The book does not have a book cover, so it never appears on the Smashwords front page. It also spends most of its time unpublished, and in my archive, so it doesn't generally appear on my author page. But whenever I'm finalising a new book, I dig 'Do Not Buy' out of storage, and update it with my new work. I always get the Autovetter error 'does not have a cover', of course, but I'm not bothered about that. What I then do is download the new version and check out the formatting. Once I discover the inevitable errors, I fix them, rinse and repeat. Only once everything is perfect do I publish my new book for real, this time with a cover. 'Do Not Buy' gets retired back to my archive and hopefully my new readers are suitably unimpressed with the flashy way my chapters work properly, the fact that the fonts are all correct and that the book starts in the right place. Because after all, it's the content that really counts. And you don't want your readers distracted from your brilliant prose because the formatting is messed up.