I think both are myths. I have been using a simple IDE cable witrh CD-ROM and HDD connected directly to the motherboard of my A4000 for years without any damage. And I also use a switch on a multiple socket outlet to switch all machines (computer, monitor, printer, external drives etc.). Switching *off* should not do any harm at all. Switching *on* all machines at once sometimes blows the home's fuse, but does not affect the computer's health in any way.
Bye,
Thomas