First, let's remember that the goal is to minimize exposure to NTM, we will never eliminate it entirely.
My filter canister is below the kitchen floor in the laundry room, in the cold water line, about 3-4 feet from tap itself. The small amount of NTM that could be in the pipes "upstream" of the filter is vastly diluted by the volume of water coming through. If we are gone for a while, we run every tap for 5 minutes, flush every toilet several times to clear the taps per Dr Falkinaham's recommendation.

Dr Falkinham's studies of showers with hot water at 130-135F or above also notes that the amount of NTM exposure from the cold water mixed in is very small.

I cannot speak to the issue of showerhead exposure, but using the above info, I believe that exposure would be small.

Here is my logic - my "rainwater" shower head doesn't produce much mist, the concentration of any NTM is negligible per Dr F, most of the water drains/drips out after use allowing the fixture to dry, so it is an acceptable level of risk to me. If I was a more cautious person, I would possibly soak the head once in a while to eliminate biofilm.

Each of us must choose our own level of risk tolerance. After 5 years with minimal exacerbation and no return of MAC, this is what works in my life.