Right magneto is "hot" (ungrounded). The "OFF" position grounds both
mags.

That was my understanding too, but after arguing the
point with two very experienced pilots yesterday, I
wondered how prevalent the contrary view might be.

Most pilots don't understand much about aircraft systems,
and many who have automobile maintenance experience don't understand
magnetos. They make a lot of assumptions and sometimes harbour many
misconceptions. A magneto's primary winding is grounded to kill it, so
when the key is in the R position, the L magneto is grounded, and vice
versa. Even the terminals on back of the switch are labelled R and L
and when the appropriate positions are selected, the other terminal is
grounded. Can confuse someone trying to test the switch itself unless
they know what the principles are.
Another misconception: that the magneto is somehow connected
to the aircraft's electrical system. They're afraid to turn off the
master switch in flight lest it kill the engine.

I know from personal experience that a lot of pilots continue to have
serious misconceptions about such things, but it still bothers me. This is
the sort of thing that is part of every run-up, and therefore should have
been part of the aircraft systems knowledge requirement to get the
license--not necessarily the grounding issue, but which magneto actually
fires its set of plugs.