Transmission fluid is normally checked with the engine idling and fully warmed up. The gear shift should be in neutral or park, depending on the car, with the handbrake on of course.
After a long fast trip, I agree that you should let the engine idle for a few minutes to normalize the operating condition. The fluid will still be moving around, of course. Some cars have a “cold” reading on the dip stick. That’s handy, but the engine still has to be running.
When checking ENGINE OIL, its necessary to shut the engine off and let the oil run down to the bottom of the engine. This will take no more than 5 minutes if the engine is warmed up. Alternately, check the oil before starting up the engine. That will show the true level, since all the oil will have settled in the bottom overnight.