I am wondering…
I have an Oak D that has some burrs at the end of the shaft, they are very minor but they are perceptable, would it be worthwhile to sand these down? Also I have a Feadog that has a slight imperfection in the mouthpiece, can’t feel it with my hand but I can with my mouth, do you think this makes any difference to the sound?
Unless removing the burrs would change the length or opening of the whistle, sanding lightly with fine (320 or finer) paper, should not materially affect the sound. If the problem you have with the mouthpiece is on the outer surface of the beak (where your lips touch), again, light sanding with very fine paper should not affect it.
The things to not do, in general, are:
Make modifications which change hole diameters on the fingerholes. (This is acceptable if you understand what you’re doing and why)
Change the length or inside diameter of the main tube (other than tweaks which remove the head to make the whistle tunable)
Cut into the airway from the mouthpiece (don’t want any leaks!)
All that said, the Oak D is sufficiently inexpensive that, if you experiment and mess it up, you can always toss it and buy another. If you have an expensive whistle, though, it would probably be to your advantage to have the maker fix it for you.
Happy New Year!
~*~ 
serpent
There were some burrs on the end of the tube of my Oak. After I sanded them off, the whistle played much better.