Bob,
I'm not really sure under what (if any) conditions that applies. Apparently, it is to preclude the computer from "seeking" the peripherals during the install and somehow interfering with the install. The rationale is that even with the devices turned off, the computer will seek peripherals provided there is a device connected to the computer. Ergo the physical disconnect, ideally at the computer end and not at the device.
Far over my head/beyond my depth am I. Just because I read that somewhere does not make it so. Even MFI had posted "incorrect" advice.
All said, I do it because it is easy for me to do and can't hurt...and it may just help.
Perhaps some clever chap or chappete, who
really knows his/her stuff will chime in.
I upgraded to 10.4.8 with Combo Updater (have always used Combo, never Software Update…) and have had no trouble. I have not bothered to disconnect peripherals (external firewire hard drive, router, compact flash reader) for a long time. I think the disconnect peripherals relates to the problems that appeared with some, but not all Firewire 800 drives which got corrupted during updates in the early days of Tiger. It is pretty difficult to perform an update of a cloned system on an external hard drive without the drive being connected.