If you jailbreak the iPhone 4S?

If it is an untethered jailbreak and the power button breaks as it normally does with my iPhones, will I be able to return it? I have insurance and often return the old phone and they give me a new one, but I was just wondering if the jailbreak would affect that

Nope, your warranty will b void and apple will not fix or replace.
you can't get rid of jailbreak files no matrer how many times you restore and apple can tell in 2 secs it was jailbroke