Once tuned most gassers require no further corrective action for day to day. However, many engines have an engine installed with the spark plug at the lowest point in the engine, others, have a downwards slanted plug, and others have an outwardly direction plug. All of those capture residual oil and fules from the last run. Some sit for extended periods of time after the last run. Unless you change plugs every run the only way to clear those residuals is to get the engine hot enough to burn them clear. A partial power run may not generate enough combustion energy to clear residuals from the engine.

Never, ever, take off with a cold engine for the life of your plane will then be greatly jeapordized.