One thing to keep in mind is that mirror position affects how you will be able to focus on the subject. Somewhat surprisingly any SLR type cameras have a rather fragile mirror support at both top and bottom. If you apply pressure to the mirror in order to clean it, it can affect where the mirror rests. What that would do is, it will change what your view finder will show that the image is in focus. It will then not match when the image will actually focus on film. That will, in turn, make it so that if you focus on the image on your view finder, the image is no longer focused on your film = blurry out of focus image.

Try this.... put on a lens with a widest aperture and the longest focal length. Open the lens wide open, say f/1.8. Put your camera on a tripod. Focus on a subject near by very carefully. Snap a photograph. When you develop your film, take a magnifying glass and examine the negative. Is it in focus? If so, you are fine. If not, you need to send it into an adjustment.

Little scratch here and there on mirror will not affect anything except your pride....