In my QT tank I wanted to go completely bare bottom, but I'm not crazy about the look and I have read some posts about goldfish being confused with their reflections. I had already used a brilliant aqua blue to paint the back of the tank, so I painted the bottom as well (the outside of course). With a few plants in glass bowls and some gemstones this makes a lovely main tank as well.
And for those that aren't crazy about the idea of putting paint on their tank, you can cut a peice of foam to the perfect size to fit underneath your tank and paint this foam. You can get creative and paint a mural if you want. This serves two purposes. The first is that it helps bear some of the water weight of your tank and more evenly distributes it (something that is a good idea to do for any set up) and secondly it adds a nice coloring to your bottom if you paint it and makes the fish feel more comfortable.
If you click on this link you can get a bit of an idea what it looks like. The jar is sitting on top of the painted blue bottom.