Yes Rock, having come from a traditional background I keep a finger over my arrow out of years of habit. Now as far as the inconsistency question, as Bronko stated, I consider when someone smartens up and replaces a Biscuit with something better (dang near anything) and I see their groups improve dramatically then yes I have to assume it was the part. When using most all carbons, those bristles get worn if you are shooting as much as one should. Not to mention the fletch problem. Wrinkled up fletchings cause poor flight as well. Same thing with "horizontal wear" of those bristles. No way they could wear evenly so you have one side weaker than the other which will turn an arrow. Again, all this is based on shooting as much as one should shoot throughout the year to remain on top of your game. Sadly there are too few people that actually practice that much.
Maybe what you and many others call "shooting fine" is quite a bit farther from my definition of "fine". If I'm not busting nochs at 20-40 yards my equipment isn't tuned properly. Call me a perfectionist (it's actually quite accurate) but I am of the same mind with my firearms and my archery equipment. If I can't tune a round to at LEAST MOA for a firearm then I work on it till it can.