If possible, the actual switch position should be used, i.e. ON, OFF, Selected, etc. Many agree that "as required' really says nothing and communicates nothing from the pilot responding to the pilot challenging. Also, the Pilot Monitor (old Pilot Not Flying) should check the switch position is actually in the position called out. More than a few incidents/accidents have occurred with the pilot saying the right thing, the switch NOT being in the correct position and no one noticing.
It can get wordy but where variables are involved, the actual settings *may* be called out. For example on Radio/NavAids, "Radio 16.9 and departure 123.57"
As noted, Degani is a great source of info on checklists as is Dr. Weiner
This link will take you to a NASA page with studies.
http://tinyurl.com/3b2d32