Hi Keith,
In my personal experience; I have found the dioptre setting on the my camera inadequate for looking through the viewfinder without wearing my glasses (vari-focals as it happens). However, I think this could be more to do with my astigmatism than the dioptre power being insufficient.
Thus I
have to wear the glasses to see the focus screen and focus points sharp, and yes, this does mean I can't see all four edges simultaneously - I have to re-position my eye to achieve seeing right to any particular edge. (I live with it)
Therefore, because I always wear glasses, the rubber surround doesn't get 'face-oils' on it to smear off on the glasses and require the microfibre cloth too often - and no-one else uses my camera! Cheers,