heres my take on it. lets say you ride board X in a cambered version and now there is the same exact board in a rocker version.
powder - the rockered version floats easier and more naturally, and personally i find them to be kinda bouncy with turns.. fun bouncy. if you size down, maybe you get the same float as your longer cambered board, but in a shorter, smaller, less stable package. why is that good?
groomers - the rockered version already rides looser and easier on groomers. if you size down, its only going to feel looser and smaller, and again, less stable. why is that good?
the only scenario i see making sense is if you were sizing UP for pow with cambered boards from your normal size. so maybe with rocker, you can come back down toward your normal size, and get the float you were looking for by sizing up, but without sizing up.
in general i say stick with your normal size. rocker isnt there to allow you to change sizes, its there to be a different ride.