If the floor is not flat, it will make it quite difficult to get the tiles to lay evenly and flat. From what I understand, what you want is a flat floor, but not necessarily a level floor. So, as long as you can smooth out the floor so that a straight edge lays evenly flat against the floor, tile will lay correctly - so the floor can slope, as long as it's a straight slope. If you can even out the surface by filling the dips, the slope itself shouldn't be a problem. The best means of doing this depends on your subfloor material. There's a lot of good info on this at the John Bridges website, and Bill Vincent did a tile FAQ over on the Bathrooms forum that's also got fantastic info.