Yes, that is correct. If a state’s transitions mode is set to “Check Continously” it will check all transition condition per-frame while the state is active. Thus you are also correct that when that is the case, you should probably also reset the “idleAction” to empty so that a transition is not made immediately.
There is however a window of 1 frame before transitions are checked. This means that you can if desired, reset “idleAction” to empty within your “Idle” state, yes. Just make sure that it is the first thing you do in the state (meaning doing it in the 1st execution frame) 🙂