SmokeAnd Noise,
EGPWS does radiate, it uses the radio altimeters.
Re the safety: the 50W to 100W quoted goes into the pencil beam in the front of the aerial. All you get behind the aerial is a few milliwatt spillover.
And if you're inside the aircraft, all of that gets reflected by the aircraft structure, so it doesn't even get to you.
Bears absolutely no relation to the 1W to 2W that your cellphone pumps out right next to your ear and brain.
Re switching it on and off:
My experience with all kinds of electronic equipment is that most failures are caused by the transients when switching it on and off, not by keeping it running.
The only reason I can see for switching it off might be to reduce wear and tear on the mechanical bits (antenna pedestal), and in old models that still use tubes to prolong the tube life (although they don't like switching off and on either).