The tile contractor noticed that the bathtub wasn't totally level (about a 1/4" off). That didn't seem like a significant amount so we decided to ignore it thus learning a very important lesson--water doesn't flow uphill.
Unfortunately, the tile had been installed by then. So, leveling the bathtub involved the messy procedure of removing the tile, removing the cement backer board behind the tile, disconnecting the bathtub from the concrete block and cutting a hole through the sheetrock so we could put a lever in to raise the tub and put shims underneath. We still have to pour a liquid concrete mix underneath so the tub is supported.
About the only good thing to come out of this was that I got to play with a new tool--an angle grinder with a diamond blade. It is amazing how that thing cuts through concrete, tile, block, steel--truly a great toy.