Granted, but they do give a nice sense of variation. I never used skins until the last week or two in all my years of playing.
I do find, however, that I do better, or worse, with specific skins even though the weapon fires the same. This could be due to the mentality that the sound or look gives, influencing you. As an example, Valve's glock sound is just known as weak, but if you use a skin that gives the sound a POW, it makes you think higher of the gun, and actually makes you do better with it.