To tell you the truth ever since I put a case on my new iphone 4 I never noticed that signal fluctuation no matter where or how I hold it now.
So it really does help.
Yea from my findings, it has to do more with how far you hand is from the antenna, more closely, how close to the gap between them. I can get it to lower bars if i put my finger close enough but not touching it. More then 1-2MM's away though and it doesnt mess with it.