When I had my interview, the consulate officer asked what my wedding plans were and I answered honestly -- that we haven't really been able to plan much due to the uncertainty of the K1 timeline, but that we were planning on having it at city hall with some close family members and friends in attendance. She didn't ask for any further information or proof, and I was approved.
Since this visa makes it very difficult to plan anything, I don't think they really expect you to provide concrete evidence of a venue or wedding invitations. If they ask what your wedding plans are, being able to describe what you guys are thinking of should be satisfactory, but it wouldn't hurt to bring along screenshots. Sounding unsure of your answer, or saying something like "oh we haven't really thought about it, we'll just figure it out when I get there" would likely seem suspicious.