It can be tricky to select the correct diopter, but waileong is right: the built in optics of the viewfinder system is just right for perfect sight. Any deviation from perfect needs full correction. Ideally you should try your optical prescription and +0.5 and -0.5 in dim light and select the one that's best.
A complication with rangefinders is that you are actually looking at the subject through an optical system and you still need some focusing accomodation in your eyes to focus on different distances.
This is different to an SLR where you are looking at a ground glass screen that is always at the same optical distance from your eye.
The upshot is that when you are oldish, and your eyes have very diminished accomodation, you need to select the diopter value very carefully. Dim light makes for better discrimination between the values being tested.
Data points...I am not an optometrist. My prescription is in the range of +1.5 to +2 depending on range. On my M bodies I have +1.5 (preferred) and +2 diopter lenses and they all work fine. YMMV, but I am getting to the point of "oldish" and can accomodate that variation. (The same is true on my <gasp> Nikon SLRs, where I use +0.5 lenses usually but can use a +1.0 just fine.)