The Apple LaserWriter 12/640 will print a Startup page when powered on whenever the Communication Switch is in the "reset" or "factory default" position, that is the switch is in the "out" position.
If you do not want a Startup page to print when the printer is powered on, place the Communication Switch in the "normal" or "customize" location, in the "in" position, then via the Apple Printer Utility uncheck the checkbox for "Print a startup page each time the printer is turned on."
Note: When you try with the Apple Printer Utility to disable the Startup page when the switch is in the reset position, this dialog is displayed:
Warning:
Some settings will not be saved on the next power cycle. (See switch position)
If you receive this message, place the Communication Switch in the normal position sometime before restarting the printer to not print the startup page.