I'm going to be installing a porcelain enamel/metal bathtub (American Standard Cambridge) and the instructions do not call for a mortar bed but I would like to do one just to be safe. Will this harm anything?
Contact of the bottom of a steel tub with a mortar bed could cause corrosion of the tub from the bottom. If the instructions do not call for a mortar bed, I would not use it without expert opinion to the effect that it is a benefit. A good quality steel tub does not need the support of a mortar bed.