We use only no-dye-no-perfume liquid soaps and they rinse very cleanly. I've never seen soap scum on our bathtub and it sure isn't because we scrub it. (Unless J and X have secretly been scrubbing it all the time and not telling me.)

I take baths with powdered oatmeal (SO NICE) and rinse the tub (and myself) pretty thoroughly afterwards because the oatmeal can get quite slippery. Beyond that... uh... I guess I give it a quick scrub after washing out the catbox every couple of months?