hooked up... parking brake set on TV...
Unless it is chocked (with good solid x type chocks) and landing gear is down... then I would do it off...
I would never ever take it out without one of the two above.. you get the tongue light enough it will take off on you.. simple physics...
when you see a tailgate with a dent in the middle of it about 1/2 way up that is where they tried to put something on or off a trailer without it being attached or secured....
I know this because one time I was loading a car and my "helper" thought to unhitch it while I was bringing the car around so it would load easier.. to his surprise when I stated up the ramp up came the tongue and he was trying to stop the trailer when I hollered get the blank out of the way... and poop it went into the back of the tailgate... lesson learned for him... me well I traded that truck off later