It's not really worth worrying about; first, all of the dedicated readers--Nook, Sony, Kindle--use a non-proprietary file format. It's not the same one, really, but that doesn't really matter.
Secondly; all the smart phones and tablets can use multiple file formats/reader apps.
Think of it in terms of paperback and hardback and trade; just different containers for the same book. You want to have as many containers as possible, since people may have a preference.
The few extra hours it takes to properly output a specific format are trivial in terms of the potential sales, if the book sells.