Generally the only way to do this is to turn the volume down the last time you used it. The boot chime is part of the firmware, it exists independent of the OS.
Still, there's not all that much different between 10.5 and 10.6 really. All 10.6 was, was a cleanup and optimization of 10.5. They dropped PPC support, finally got rid of a few depreciated APIs, and did some performance tuning. Virtually every tip and trick that worked in 10.5 will work in 10.6. Just repeat what you did earlier.
I like to use my iMac 24" at odd times and wish to mute the startup chime (or at least make it quieter) so I won't wake up my spouse. Apple Tech support helped me do that when I had 10.5.8 but they deleted several cache files buried deep within the OS. My tech support has expired and I wonder if any of you can help?