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First up, if you are happy with your results without flats, no one should step in and say that you MUST take them.
From my own equipment, both scopes use a ZWOASI2600, one MC and the other MM. I find flats essential with my newtonian both for any dust bunnies that creep in, and it has some vignetting related to the secondary mirror. My refractor on the other hand has quite a flat field illumination wise over my APS-C sensor, but flats do still make a difference that is visible. I always use flats calibrated with a master bias (I shoot sky flats so exposure times can vary from about 0.5 to 20 seconds, dark-flats would be no good to me) and I calibrate the lights with the master flats and a master dark of the appropriate length. I find without the master dark that the master flat tends to introduce brightening in the corners as the master flat acts on the dark current related "signal". It is minor but the longer the exposure the more pronounced the effect.
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