Apple's 10.5.6 Combo updater page states that OS X 10.5.
1 is required:
http://www.apple.com/downloads/macos...mboupdate.html
Don't know if that's a typo, or if 10.5.0 users must first update to 10.5.1 before applying the 10.5.6 combo update. Typically, I keep the latest combo updaters around for new installs of Mac OS X -- I can quickly bring a system up-to-speed in terms of updates by applying the combo updaters immediately after installing OS X clean.