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My understanding is that if you get the tilts of the mirrors right (and the rotation of the secondary) then you should get sharp image which is the first thing you want. Any probably that is the most important outcome you want, to start with. If, however, you are achieving this without seeing the full primary mirror reflected in the secondary, then you are likely losing some of the light that is being reflected off the primary. The effect of this is to slightly reduce your limiting magnitude??
Maybe this is too simplistic an understanding, but I think, initially, you want sharp images. Trying to get every photon that is captured by the primary and excluding every unwanted photon (with flocking and light shields) can be secondary activities.
In the latest Oz Sky & Telescope, there is a review of a secondary mirror holder where the pivot point is almost at the back of the secondary mirror. In the GSO, for one, the pivot point is cm behind the secondary, so tilting the mirror can move it significantly. That is probably what is happening. I understand then that one should follow an iterative process - tilt/position/tilt/position/tilt...
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