The danger with any of the mag "dead checks" is switching off and then not
immediately switching them back on. Even a slight delay will result in a backfire and probable damage to the muffler and/or other parts of the exhaust system.
Personally I think that the after start "dead check" is not worth thr trouble as it will only detect a totally dead mag which is a relatively rare failure. I have only had one mag fail (and it was in flight) in 35 yrs of flying, so the benefits of the after start check seems extremely limited to me. In the case of my mag failure it would not have mattered as I detected the failure on the shutdown check.
As for the shut down check, I just set 1000 RPM and cycle the mags Both/Left/Right/Both. You will be able to see the mags cutting out and the chance that the mags will cut out normally, but be live with the mag switch I the off position is extremely remote.
One point that IMO, is insufficiently emphasized at flight schools is to check the mag switch key lock. The key should not be removable at any position except off. Therefore when I cycle the mag switch during my shutdown check, I give the key a gentle tug at each position to make sure thr key does not come out. I have twice seen worn mag switches that allowed the key to be removed when the key was selected to the right mag (the one next to off on most switches)
. This IMO is the most likely cause of a hot prop and is not tested for in the traditional shut down "live mag" check.