Yes, you are correct not all National Instruments Analog Output devices will continue to output the last updated value after cleared. However, I can confirm that all E, B, S (61XX), M (62XX), and 67XX devices will continue to output the last updated value. The channel property Idle Output Behavior for Analog Output is not a supported property for the devices you have listed. I have seen this particular property used with a Modular Instrument (MI) - DSA device, which is one of the NI Analog Output devices that do not always continue to output the last value that was updated.
This question is very application specific. Just be sure you are aware these DC values exist when making your Engineering decisions. As long as your application functions as you expect it too, even after you clear the task, then I do not see a down side to allowing these DC signals to remain. Have a great day and good luck with your application!