They may or may not refuse to service it, depending on what the issue is. If the jail breaking interferes with their diagnostic software, or is potentially a cause of the symptoms, they will send you away.
They will ask you to restore your iPhone. That's it.
Policy concerning jailbreaks changed in 2015. They don't like it still but they won't refuse service.
Pretty sure it voids your warranty, so technically I think it would be within their rights to refuse to service it.
Yes, that's what they say. And if they want to be real jerks they can probably push it. OTH, 99% of the genuises working the bar just don't give a ****. They're behind and truly do not care. Plus, the in-store policy is that if a JB is not interfering with the device (and causing the problem it was brought in for) it doesn't matter.
You will be asked to restore your device, which removes the jailbreak and their problem.
Why not just remove the JB before you go in?
Exactly. Like a lot of seasoned jailbreakers have done for years now.