In theory the idle RPM should be set at about 650-700 RPM, in reality I find its usually about 850-1000 RPM. My basic run down procedure is to idle the engines for about 3 mins at the lowest RPM that they will run stable at, check that the EGT is below 800 degrees, then shut down. Running the 'Hi' fuel pumps during cool down will also lower the RPM by about another 50-100 RPM.
The mag check at TOD is a useless and perhaps even dangerous idea, I've had 2 mag failures in flight in Navajo/Chieftains and trust me you notice them, symptoms include slightly rough running, a small RPM drop and a rise in EGT, all of which are
noticable during standard CLEAROFF checks, thus no need to do a mag check at all during flight... would be VERY surprised if that crap was in an ops manual.
As for taxing at 1200 RPM... not a standard operating procedure. I often fly night ops in the Chieftain and have never had a situation where I have run low on electrical power or flattened the battery during taxi, just operate as you would normally duing
any other flight.
In short, every operator has different procedures, my advice is to listen to all the available advice/theories then adopt what works best for you (With-in the bounds of the company ops manual, of course)