Turning off the mags totally is so daft that it's unfunny. All sorts of things can get blown to pieces like that.
Mag checks should be done to allow the engine to stabilise at the RPM rather than just a quick drop for the reason Keef has given.
If there is one thing that drives me nuts, it is having people rushing through their checks, the Carb heat being a particular favourite.
The mag check just after start is only to check that you can actually switch the mags off. It isn't a meaningful check of RPM drop since the rev's will be low and the engine isn't at all warm.
The RPM that you check the mags at is different depending on the type you are flying, eg. C152 1700rpm, PA28 2000rpm, HR 200 1800rpm etc..... The POH will have the right numbers in it for the machine you are in.