I did that, but I have 2 partitions on my external drive. One is still on 10.4.11 from the date I upgraded, and the other is my Leo backups.
I encountered a few things that didn't work - a Brother printer driver, the software for my Logitech wireless keyboard, and worst of all -
the complete inability to download files with Firefox 3. Safari also needed an upgrade. I suggest you read the stuff on Firefox (if you USE Firefox) before you do the upgrade and be prepared to remove the 2 .plist files
before you start the upgrade process. Maybe even color them for later. They will have to be located and removed after 10.5 is installed.
A good source of Leopard conflicts and solutions is on
http://www.macosg.com (site is down today).
Also, as discussed here, when you boot from a Leopard backup, the startup disk may not appear where you expect it (although your Tiger backup will). This is an annoyance, to be sure.
There are a couple of features in Leo that are marvelous.
Also I recommend David Pouge's Missing Manual - Leopard book. Apple doesn't tell you anything.
FWIW,. my own upgrade failed in the middle ands the DVD popped out. When I reinserted it, right after acceptance of the EULA, it picked up where I left off. I don't remember selecting HOW I wanted to upgrade.
After everything was running the way I wanted it, I SuperDuper!'d my new Leo drive via erase and copy, not smart update, but that's just me.
One more thing -
repairing permissions the first time took over an hour; now it's down to about a half hour. If you repair permissions when you run SD, don't think it has frozen. It just takes forever. This is a known Leopard issue.
Have fun.