I've been doing upgrades for the last 4 years (Jaguar->Panther-Tiger so far), and I've never had an issue. I have one machine that by the end of next week will have started off as Jaguar and ended up Leopard, and I expect no problems.
Archive and Install if you're worried. Doing a fresh install is too much hassle for me because I have so much stuff on my computer I can't just wipe it off to upgrade an OS.
I can't help but feel all the people who are "really recommending a wipe and fresh install" are coming from years of dealing with Windows.