When gaining my licence, it had always been explained to us both by the various FI's and our powerplants groundschool instructor that the initial 'dead cut' check of the mags after starting was just that - a check to see if the individual mags were 'dead' or not. If the key (or switch) was turned to select either left or right mag and the engine died, we took our hand off it immediately and allowed the engine to stop before calling for an engineer. The check is always carried out at low RPM as this is the situation which would result in the least damage to any components in the event the mag was inadvertantly turned back to both and the engine restarting.
The other mag check at higher RPM is to check that the RPM drop from both mags to single mag operation is within limits, and that the engine would continue to function in the event of losing a mag in flight. As has been said, during this check the RPM should be allowed to stabilise for a few seconds on each mag to check for fouling of the plugs. There should be little or no risk of a dead cut at this stage which could damage the engine as that check has already been carried out.