We just had the engine guy here from the USA before Christmas rebuilding one of ours, and i asked him that very question. The answer depends on the design of the engine. Some engines have a flight and idle setting, and the oil and bearings need the chance (and are designed to operate at an idle setting), to circulate the oil and to cool down. In the Astar, the engine has no idle setting and the bearings are designed to be lubricated when the engine is at flight. Turbomecca recomend 30 seconds at 70%NG to allow temps to stabalise and then shut down. Running the engine for longer than this means the bearings are no longer being lubricted properly and can cause coking and damage to the bearings. Thats what we were told anyway, open to other advice or thoughts though?