Mirror flop on SCT's is an urban myth...
There's confusion about focus shifting and the mirror flopping around on the baffle when the scope changes orientation across the meridian.
I asked this question on another forum almost three years ago and requested copies of images showing definate mirror flop.....I'm still waiting.
I think a more sensible answer lies with the guide set-up flexing....
You can run the main mirror up and down the baffle a couple of times to minimise focuser shift - I do this when I remember.....