I think that the top surface should be level. If you do that, you should find that the actual inside bottom of the tub is pitched down to the drain. Depending on the tub design (fiberglass, acrylic, cast iron, etc.) it may have a base designed to sit on the level floor. If not, the best way to set it in place is to use some mortar. The mortar also helps if the floor isn't actually level (common). Most tubs are also supposed to have the edges supported by firring strips or a platform (if it is a drop-in). The tub should have come with instructions. If not, call the manufacturer.