My first instinct would have been to visit the Watch Manager in the Tower upon shutting down, whilst the event is still fresh in everyone's mind. If you were 'cleared' to taxy, it implies full ATC. You mention Tower and Ground - this only occurs at very busy ATC aerodromes.
Worthy of a report to CHIRPS - GA section. They will investigate confidentially and report back if you feel the moment has passed or you don't want to discuss it directly with the ATC provider.
My personal view is that the downwash from a helicopter is equivalent to the weight of the machine. If that were to hit one of your wings, it could quite easily tip that wing to the ground. There's NO WAY, in my opinion, that a helicopter should hover-taxy over another aircraft. I'm surprised, actually, that a helicopter is taxying at 20' AGL.I'm not a helicopter pilot, but in my experience they normally taxy within a foot or so of the ground, in case the engine fails. At 20' there's no time to autorotate, I'd have thought.
In any event, you did precisely the right thing. If in doubt - STOP.
TOO