White back replacement

Hey guys I have reading alot here. Got a question

I do know that if I replace the back of my iPhone 4 to the white one w oem or some cheap eBay one it does void the warranty and I understand that. But before taking it back to apple store if I ever run into a issue with it. Can't I just put the real "black" back on it and make it look like new?

I mean in my past 3GS experience. They never look at the phone in detail when replacing it out. I mean they do look at damages and water exposure. But are they really going to take the back off and check it?