Does the VOOM STB need to be on (vs off or standby) to get the channel updates, etc?
Is it better to leave the STB "off" at night or just leave it "on"? (We have it plugged into a UPS)

For channel updates it doesn't matter. For software updates (that don't happen too often) it is better to leave the STB off (in standby) at night. But even if you leave it on all the time, you will get the update sooner or later.