This is a pretty broad question. It falls in the same category as "How long is a piece of string?" and "How long does paint take to dry?". Without more details, nobody can answer it. And even with more details, you will still get different answers from different people depending on where they live. If you are suspecting a little plastic case will affect the reception of the phone, you might as well ask what type of shingles are on their house and if the have a sunroof in their car, since the signal has to go through those to get to the phone.
I personally have a cheap ebay silicon condom on the phone and a screen protector. The phone gets great reception through the case, but I think the screen protector is screwing with the proximity sensor as the phone doesn't seem to realize when I pull it away from my face while on a call. (Asphalt shingles and no sunroof in my car).