to a tone freak, yes its true, but to someone that relies on common sense, its false.
alot of people during the 70's thought that paint jobs hindered the guitars ability to correctly reproduce the guitars tone.
now we realize it was just all the cocaine that made people think that.
by taking off the back plate you won't be changing the sound or "tone" of your guitar.
are you referring to the neck plate or the trem cover?
most of the time the trem cover is removed just becuz its easier to re-string the guitar.
the idea of a neckplate is to have even bite on the neck to the body.
if you remove that you will risk pulling one side of the neck more than the other as well as messing up the finish.