That's another thing that worries me. I answered "No" to the "Have you ever been cited..." question. Like many others, I didn't think this applied to traffic violations. I have proof of payment/dismissals for most of my traffic violations, with the exception of a couple. I know I should be as honest as possible and let the IO know that I've had traffic violations, but mentioning a violation for which I have no paperwork seems like opening of Pandora's box.
I think the worst thing that can happen is for the immigration officer to put "pending" on application after interview if you don't have evidence of payment, but once it gets forwarded to a supervisor they will approve it (without requiring evidence). I can't see USCIS denying an application solely on not being able to produce evidence of payment for citations under $500. Now if the amount was much larger and you had several citations, that's a different story. It's a gray area in adjudication as every case is different and they look at all the facts when deciding to approve or deny an application.