The guy in that link gives good advice. Think of it this way...
- The simple advice to a child is "Don't put things in your ear!"
- As adults we might put a Q-tip into our ears in an effort to clean the insides, and most of us know not to push too hard and too far.
- But even with that cautionary advice, sometimes we screw up and need to make an appointment with the doctor's office.
- And when we go into the doctor's office, he takes a look and then deems it appropriate to tell his nurse or medical assistant to irrigate our ears to remove the wax we pushed in, and then he leaves us.
We do NOT need the doctor to irrigate our ears!
Cleaning a mirror is like the various stages of care and cleaning of ears. It is far simpler to avoid problems with the "DON'T do that!". But educated with appropriate techniques and equipment, even WE can clean our mirror surfaces without sending the camera 'to the doctor'.
The reflex mirror has a very thin vapor coating on its TOP surface, to reflect light UP to the viewfinder while also sending light THRU to the AF sensor. If we screw up the surface coating, we might not only degrade what we see in the viewfinder, but we also could affect the AF accuracy. It is far simpler to avoid problems with the "DON'T do that!" than to educate every consumer how to do it the correct way.
Since reflex mirrors never affect the IQ of the sensor, the best advice is to simply leave it alone! When the OCD is too severe to bear any longer, clean the mirror...but otherwise simply leave it alone. same advice works relative to the focus screen too... never affect the IQ of the sensor, so leave it alone. When the OCD is too severe to bear any longer, clean the focus screen...but NEVER touch the surface of the screen because you WILL visibly screw it up and make your OCD even worse, to the point of having to replace it. (yes, there is a way to clean even the focus screen, but that is off topic of this thread!)