IIRC the spherical aberration with the EdgeHD series is minimised at 146mm on the 9.25 and upwards. If you stray inward or outward from that, it increases at the edge of the field, but not very rapidly.
I remember seeing plots somewhere, a few years ago, and thought that it was a lot less dramatic than expected, even over deviations of 5-10mm from stated and it's mostly noticeable off-axis and exacerbated with a full-frame camera sensor.
In practice, you might see stars looking more fuzzy than sharp, as you stray away from the recommended back focus, but my take on that is that given the focal lengths of the 9.25 and above, you're prolly going to get some ambiguity in the stars because of the pesky atmosphere
With field flatteners used in refractors, incorrect back focus is very noticeable except with the smallest of sensors, and coma correctors used in newtonians tend to be pretty fussy too.