Sure is a lot of litterature and different opinions on the corrector plate placement. Some people go as far as saying the corrector plate and mirrors are mass produced and not necessarily matched, just generic.
My gut feeling is that the corrector plate and the primary and the secondary have to be parrallel and also not rotated vis a vis one another. Having said that I'm am not convinced (dont' really know but) that a slight offset in the corrector plate axis/secondary and primary is a "picture killer". Having the corrector plate not parallel to the primary would be very bad I would have thought though. It's like your camera not being squared. Your field looks like
I'm also hearing that not having concentric shadows when looking at the front of the OTA or the back with a draw tube/cheshire is not a concern because of the SCT design and doesn't mean it's not aligned. That I'm not buying though
It doesn't make sense to me. Anyone disagree or care to explain why it wouldn't matter? I don't get it.