You don't have to worry too much, but it isn't a good practice to let it sit for 20 minutes at idle without airflow across the cooling fins.  In that case the oil is the only coolant, but generally it isn't too much of a problem.  It is a matter of common sense.

Now, what I have done many times in such a situation, is to place a fan on the floor at the front of the engine...on the exhaust.  This will allow you to tinker with it while it is running without worry....you can even feel the heat being carried off the engine too.