Understood. I didn't know that, and will bear it in mind. Thanks for the clear & concise reply.
I do have two friends I put onto SuperDuper! whose nightly schedulings under 2.7.5 have run without incident on their Snow Leopard Macs for for a long time. Maybe they've just been lucky.
I googled around for reports on when
launchd may be unreliable on Snow Leopard, and I found this on MacTech:
MacEnterprise: Snow Leopard, launchd, and Lunch
Thanks again for the heads-up, David!