Well, unless you have glasses and your eyes are getting old and frail you probably don't need an eyepiece diopter. If this does apply to you then you would probably want one that approximated the diopter of your glasses. This is usually done because its kinda hard to look through some viewfinders with glasses on as you can't get your eye close enough to see the whole frame.
At worst you can rig a mount to use the lens as a closeup add on.