I vote for Edward's idea. I have several test WP installs on my local testbed. One is the latest trunk version of WP, one is an exact mirror of my current web site, and one that I use for theme testing, which has the latest version of WP, an XML file that has some content in it and various plugins that can be enabled/disabled to test with various themes.

But then, I'm needlessly obsessive. That said, I find it impossible to really test or customize a theme without having SOME existing content.