I have skinned at least a dozen bows possibly many more, I don't remember all of them. I would make every effort to get all the scales off. On some of my earlier skinning jobs I would miss an occasional scale and like Dano said, its coming off somewhere down the road and leaving an unfinished white spot.
I use duct tape on rattlers and masking tape in copperheads. Careful application and removal of the tape is required, I have never torn a skin. I also do the scale removal after the skin is glued on and dry, not much chance of tearing a skin after it is glued on.