Hi MissJenna
I just did some testing. Im using a Silver "metal" back (see pic below) i purchased from ebay. I put metal in quotes since Im not sure the back is all metal or just has a metallic finish.
Just tried the Signal app (Cydia) and the readings I got were pretty much identical when I changed it to the Apple glass back.
I didnt hold the phone while taking the readings. So Im not sure, if holding the phone would make a difference, but by itself the metal back seems to have no affect on the signal.
Pic for reference: