Owenlars, when the mags checked at start up, then you cannot really tell if you have one fouled up plug. The pre takeoff power check is the the best place to tell if you have any problems such as a fouled plug, since the engine should be coming up to temperature and the great gobs of fuel you chuck in at start up should have disappeared by then.
A fouled plug isn't a problem really. the vast majority of times it can be burnt off by increasing the rpm and leaning the mixture for a few seconds. Get your FI to show you if they haven't already.
A SSD says, switching an engine off by the mags isn't going to harm it, it's the unpredictable nature of what happens when you turn them back on that's the issue. If you have a motorbike. try running the engine up and then hitting the kill switch momentarily, see what happen next! Either a flame out the exhausts with an almighty bang, or a kick back through the carbs leading to singed privates. Now try that in £100,000 a/c!