I've caught mag and plug problems at the runup though my most onerous mag problems NEVER showed up in a preflight mag check (you could do 100 mag checks at various power settings and for some reason as soon as you got 400' in the air they would start misfiring).

I assume run up means any preflight check you do at a power setting other than near idle. On my plane the prop cycle and mag checks are the only things that need be done at that power setting. I will do the mag check pretty much on anytime after shutdown.