A common saw in container gardening is that you should fill the bottom 2/3 of your pots with everything from packing peanuts, to Styrofoam to old milk jugs. While this can save you money on potting soil and make a large container lighter, there is also a big downside to doing this.
When you use less potting soil, you have less water retention and as a result, a smaller margin for error in feeding and watering your plants. If you are using a good quality potting soil, and you have a garden planter with sufficient drainage, you don't need to put anything in the bottom of your pot.
Particularly if you are growing vegetables in containers, you want to use lots of soil. Most tomatoes will use up all the soil you give them, and because tomatoes need a consistent level of moisture, the more soil you have, the more easily you can achieve this.
That said, if you have a large pot, or a pot with less than stellar drainage, there are some great products on the market to fill up the bottom of your containers. Better Than Rocks, and Ups-A-Daisy inserts are two products that are well designed and really work.
So unless I’m working with a gigantic planter, or plants with very shallow roots (like succulents), I fill up my containers with potting mix and recycle my old milk jugs.