Some people have had problems migrating their users when upgrading from Tiger to Leopard. In particular, there've been some issues with permissions that have been especially troublesome (count me among those!). Even that aside, there's always the possibility of there being something migrated from the old user library that won't play well with Leopard.
Best thing to do is just make sure you have a safe, secure backup of your home folder on a backup drive before installing OS X. Backup any other important data, apps, then just do a clean install. Format the hard drive when installing. Make a new user when you install Leopard, don't import or migrate ANYTHING. Once that's done, reinstall all apps from scratch.
As for your new user, so long as it has admin privileges, you should be able to manually copy over anything you need from your backup of your old user folder. When you copy anything to the new user, the permissions should automatically be adjust to the "correct" ones for your new user.
I recommend picking up an app called
ACL Fix to keep on-hand in case you do develop permission issues. I was forced to reinstall Leopard due to unresolvable permission issues I had. Despite that, when migrating my old user data back after reinstalling as above, the weird permissions issues I was having started to creep back in. Using ACL Fix fixed that problem once and for all.
