Checks, particularly carburetor heat checks, should be done as closely as possible to takeoff. The check isn't just to verify an RPM drop, but to ensure that any carburetor ice is removed. Mag checks verify not only that the P lead is connected and the switch contacts good, but that the mags plugs haven't loaded up and we dont' ahve a bad magneto. It's very possible to have a good mag check on the ramp at the parking spot, and have the engine die during the takeoff from carb ice or fouling because of developments during the taxi time to the runway.
Correct, which is why the the carb heat check is repeated as part of the pre-take-off checks. Surely the spark plug fouling would only be an issue a after a particularly protracted taxy or hold at idle power?