Cleaning the underside of the focusing screen - top of the mirror box - is just ok if you clean it VERY delicately - reason is that it is finely textured plastic and any noticable force will leave permanent scuff marks.
A very light touch with a CLEAN synthetic brush (I use carefully multi washed make up brushes) to dislodge a dust particle is ok - I use the cleaning method shown here
The Pixel Sweeper including air blowing using a foot operated air mattress inflator for controlled pressure.
I got caught with a tiny bit of detergent residue on one of the brushes and found I could easily remove it from the focus screen with a Kleenex tissue that I folded to a point (do not touch the tip because of skin oils) and just dipped the point in clean water and then touched it to another tissue to leave the tip just moist - stroked it across the spot where the brush smear was and the screen was absolutely clean.
In any cleaning of this area try to do it with an air stream (NOT canned air IMHO) and if you
must touch anything to the screen be very gentle as it is sensitive to pressure damage which is absolutely permanent.
NOW - the MIRROR!! any dust on here will not affect the image and will be invisible in the viewfinder as it is not on a focus plane.
For the anal retentives who can still see dust on the surface and are worried by it, clean compressed air is THE ONLY THING that should ever be used if you want to have the mirror reflecting at 100%
Reason - it is a part reflecting mirror, as most of the light goes to the viewfinder and the metering sensor BUT some goes straight through the mirror to be reflected downwards by a second mirror on the back of the main mirror to the focus sensor which can be seen on the bottom of the mirror box. If you scratch the incredibly fine metalised coating on the main mirror you affect the viewfinder, metering and focus - I suggest it is best to leave it alone as much as possible.
Rod
REFERENCES -
The Pixel Sweeper Cleaning Digital Cameras - Introduction CCD / CMOS Cleaning Photo Gallery by Nicholas R. at pbase.com http://www.kodak.com/ezpres/business/ccd/global/plugins/acrobat/en/supportdo...CoverGlass.pdf