Only you can answer the question. Its simple to do.
Partition the HD in half and clone the original partition, if there is enough free space. If not, nab an external HD (don't they have boot from USB these days?) and clone. If you have time machine (which would be a good idea if you are worried about the upgrade anyways), backing up right before installation is a good idea.
Also have your 10.5 disc (a good idea anyhow).
So, after either backing up or cloning, install Snow Leopard. (If you've cloned, on the clone partition). See if everything works. If it does, great! If not, get the 10.5 disc back out and update time machine, or for cloning, simply boot back into 10.5 and erase the clone (10.6) partition.
EDIT- Curses, sputacus, you beat me to it!
However,
Wait for 10.6.2 retail discs. I just went back to 10.5.8.
^This is also a good idea.