if you're in autofocus mode the diopter adjustment is irrelevant and what you see through the viewfinder will appear fuzzy, but what the actual picture looks like will be fine.
if you're shooting with manual focus, then your diopter being dialed in properly will dictate how you manually focus on a subject.
i had this problem in my B&W photography class last semester because we were required to shoot in manual focus mode and it ruined every single photo I had taken all semester long and they all came out blurry and fuzzy, surprisingly I passed with a D+ in the class, but i was not happy at all with the grade or the photos, and it made me even madder when i figured out what the problem was after the class had ended.