Outside of the feet, what does the rest of the bottom look like? If you set it down somewhere, get it leveled so each foot is on the ground, then, in your bare feet, walk around on the inside, do you feel any flex? If you do, setting the thing in mortar will help both level it and the 'feel' of the thing. Flex, over time, can create stress cracks, and potentially at the end, failure. The most successful method is generally to set a number of piles of mortar, then set the tub in place until it is level, then let the mortar setup overnight. It helps to put down some plastic on the floor so that moisture doesn't get sucked out, weakening the mix. Some put another layer on top of the mortar to make it easier to remove the tub down the line. Mortar cures, and generally doesn't really dry, so the plastic is not an issue unless you seal things. The moisture gets chemically incorporated into the mortar during the curing process.