I've only been given the summons one time, and I had a vacation planned, I called in and they told me no problem, they would reschedule me and then they never did.
I've been told (and I don't know if it is bs or not) that they can't really do anything to you because they don't send the mail certified so they have no way of knowing if it is delivered. Not sure if that is true or an urban legend, but with the spotty state of the mail I could see it happening.
I'd advocate showing up or rescheduling. I believe it is a civic duty.