Unless you have a highly collectible piece, I would highly doubt it. I have a moon cut cylinder by TK Custom from a Model 442 that I have replaced the stock cylinder from my Model 642 with. I'm very impressed with the work that was done on the 442's cylinder. With the moon clip cut you can load the piece conventionally (one round at a time, or use speed strips or a speed loader), or use moon clips to load the full cylinder at once. I think one of the biggest advantages of the moon clips is better extraction of all the rounds in the cylinder at one push of the ejector rod. If you are really concerned about the possibility of decreasing the value of your Model 66, why don't you buy an extra "donor" cylinder and have that moon clip cut and use that cylinder while keeping the original one untouched in a safe place?

Best of luck,

Dave