More important than "flattening" the main is to hold the boom down as you ease the
sheets (while sailing off the wind).
Without a
boom vang the boom lifts up and the
mainsail twists off and spills the
wind. Close hauled the mainsheet pulls mostly downward and prevents the boom from rising but when the sail is let out the pull of the main sheet is no longer mostly downward, and the boom can rise. On your Bayfield with the mainsheet at the transom this is exaggerated since letting the sail out only a small amount changes the angle of the mainsheet.
The comments about a possible "Chinese gybe" where the boom lifts high enough to strike the back stay and the benefits of eliminating the topping
lift also apply.