Kim from London, Ontario suggests filling the bathtub with water before caulking between the tub and the tiles.
This is a great idea because by the time you add both water and people, your bathtub can weigh over 400 pounds. This can easily cause it to sink just a little bit, the floor bends, the bathtub itself bends a bit. This sinking will pull at the caulking, putting stress on this water tight joint. If you fill the tub first, you sink the tub to start with. Then caulk and leave the water for at least 24 hours so that caulking has a chance to cure thoroughly. Now the caulking will be slightly compressed when you drain the tub, but the next time you fill it and get in, the caulking will not be stressed, it will simply flex back to it's relaxed position. This avoids caulking tear away and assures a longer lasting seal.