John,
That is a very good question.
One could argue that traditional tied flies with a hackle wound around the shank DO NOT sit on the tips of the tail and hackle points as writings would suggest. I believe that they sit flat in the water surface.
So based on my belief (others with argue that this is not correct but until I see the evidence while fishing it stands) trimming the hackle underneath only removes the hackle that protrudes the surface.
Now I will even go further and say that those hackle fibres sitting below the water line act as a trigger to the trout as they are tricked into believing that the imitation is an emerging fly.
Provactive enough for you?




Having said all of that, why do parachute flies perform so well? It is because the fly sits low in the water film suggesting either an emerging insect or a fly already emerged and ready to fly off. If you study the design of the Klinkhamer Special you will note that Hans Van Klinken ties his hackles in such a way that the thorax is pushed even further into the surface.
I have a fly of my own design where the hackles are trimmed in line with the hook point. The reason is so I can 'skitter' it across the surface.
So trim or not to trim. Could come down to a personal preference.