WIthout looking up how the tub is built, it's hard to say. If it is cast iron, make sure the shim material won't compress over time. If it is acrylic, then the tub will benefit a lot from the reinforcement of the mortar base. Full of water, you could have 400-500# or more of water in the thing, then add a person or two, maybe standing, and there's a fair amount of spot or point loading...acrylic will flex. Bed it in mortar, and it won't. This helps prevent stress cracks, makes the tub last longer, and makes it feel more substantial.