Doing an in-place upgrade of Panther to Tiger resulted in a bunch of unstable apps, most probably because of corrupted 'preferences' files. The initial build (or builds) of Tiger were problematic for many (not all) upgraders.
If you have a completely solid Tiger installation you might be okay with an in-place upgrade to Leopard. As for me, various apps like Safari, iPhoto '06, iMovie '06 (haven't upgraded to the new iLife) continue to crash (Safari will often crash as it tries to start) so I'll be using this opportunity to wipe out my laptop's hard drive and reinstall everything, minus any crapware.