Archive and Install should work just fine. All those worrying about things hanging around are worried over nothing, the Archive and Install is designed for exactly this type of use.
jW
The Archive and install creates a new set of system folders, i.e., system, Library etc. what's not touched is a person's home folder. That being the case there may stuff hanging around in the home folder, like ~/Library that was SL specific which could cause issues with Tiger. Normally you go up to a new OS, not down to an older version of OS.
While it may not be entirely needed, reformatting and reinstalling will give the OP peace of mind, if things start acting off, he'll not be completely sure its related to the downgrade or another issue.