Shower Base over or to the side of tiles
You should put tub and tile backer board down on the floor where the shower will be and then once the tub is installed tile up to the edge.  You dont want tile underneath as it may or may not end up level, it may or may not come loose after weight is put onto it, and most importantly grout is not waterproof...it will absorb water and you definitely dont want that under the shower.