It depends on how much you elect not to install. You can save close to 10 GB if you really strip things down.
Things that take up a lot of space are GarageBand loops, iDVD themes, printer drivers, and foreign language support. These all can be removed pretty well without reinstalling the whole system, but some people find it simpler and tidier to just go with the complete reinstall.
That said, if you haven't used those programs, you might not want to jump right in and get rid of them. Play around with them a little bit...you never know what you might end up liking.