Our house was built late 70's, wooden floor (so Grant could crawl around underneath adding and removing cables, wires etc). We had a waterbed for years with no problem and when we build our 6ft have no pans to reinforce the floor. I think our 4ft actually has an extra pile under it as that was the space in the original plans for a fireplace.
If you are really worried, have an extra pile added under the floor where the tank is to go.