Will a kitchen with porcelain tiles on the last floor of a house add too much weight?
This might help: http://ci.camarillo.ca.us/docs/bs/Weights%20of%20Building%20Materials.pdf - it says 7.7 pounds per square foot for the tiles and the cement backing board. Which isn't that much. If you're putting new cabinets and appliances in, I'd be more worried about those. They weigh a lot more than the flooring, and the weight is concentrated in a smaller space.

Ultimately, the answer will depend on the house.