Whether you use plywood or thick-set mortar, or "mud bed," there must be a solid floor underneath before you lay ceramic tiles. If you live in an older house, it is likely to have two or three layers of wood underneath. If there are no wood layers underneath, the traditional thick-set mortar method may cause the tiles to crack. The thick-set mortar method can be more solid, but it requires a certain degree of skill to apply it and a fair amount of time to set properly. If not, you will encounter problems later on. I find that the plywood method is just as effective and much easier to do correctly. It is important to remember that either method will cause the floor level to rise, posing a potential problem with door clearance.