You can, and it may be necessary if it does not sit flat and level on your floor. Do not try to make a flat bed of the mortar...Terry likes to place a bunch of piles of the stuff spaced around, then push the tub into it until it is level. If you set the stringers perfectly level (assuming the edge is, too - it may not be!), then you can stop when it hits the stringers. Then, leave it alone until the mortar sets. Depending on your access, you may need to make the drain connection at the same time you set it. It takes some careful measurements to get it all configured, as if you need to set and make the drain connection at the same time, you only get one chance.