Risk v Reward.
In my experience, cleaning focus screens can be a pain. Even on my olympus, where the focus screen comes out easily as it is interchangeable it is very difficult to eliminate all dust. Even the tiniest speck shows as a little black dot. If I had to start dismantling the camera. I don't think I would bother. After all, it doesn't affect the picture. Unless it is *very* dusty, I wouldn't bother