I did an upgrade from Tiger to Leopard about 12 months ago, and it worked fine for me. However, I would strongly suggest to make a backup of your files, and ensure you have installers for applications, just in case you have a problem.
You can go one step further, as I did, but it does take a bit of time:
1) Restore you system hard drive to an external drive (can be done with Disk Utility)
2) Make sure you can boot fromt he external drive
3) Upgrade your system HD
4) If there are any problems after upgrading you can still boot from the external drive to then start a clean install, and choose to import settings from another drive
Note that Time Machine is excellent, but you should really be using it with an external HD. It will work with a partition on the existing system drive, but this does not protect you against HW failure.