First paragraph in the 7D and 5DII manuals on MA .
"Normally this adjustment is not required. Do this adjustment only if necessary. Note that doing this adjustment may prevent correct focusing from being achieved".
This advice is rubbish, and is presumably there to protect Canon from complaints from users who don't manage to set up MA properly and make things worse rather than better. Many lenses, especially in the telephoto range, benefit from accurate MA setting.
First sentence of Last paragraph
"It is best to make the adjustment at the actual place you will shoot. This will make the adjustment more precise".
And this is sort of half-rubbish. Setting up MA properly is a bit like doing lens testing; carefully controlled conditions and a reliable methodology are essential. It is not a task to try and do when you are shoorting. What is not rubbish is that the correct amount of MA may, and often does, depend on focused distance and, for a zoom lens, on zoom setting. If tests show that your lens needs varying amounts of MA, then use careful testing to determine the appropriate anount for various different distance and zom combinations and record this information, then make the appropriate setting when you are shooting. In that sense the advice is good.
Even better advice is that if a lens displays a varying MA requirement, or needs a large amount of MA, send it to Canon for calibration. A bit of MA fine-tuning may still be desirable after calibration, especially if the camera body has not been calibrated, but once that has been done, behaviour should then be consistent.
