Please don't do it! I was at one yesterday where there was a full bar but the bride and groom only covered beer and wine. It was confusing to the bar tender and the bar tender seemed pissed off the entire time. It was also confusing to the guests because it was not made clear if cards were accepted or not and originally we were told full cash bar. Please save your guests and just do beer and wine if you aren't going to host the full bar.