I charge by the hour, but I'm very willing to show someone at the venue the proper way to cut the cake, or anyone on the catering staff (even though one would think they would know). I generally cost too much - on purpose - to be used to cut the cake.
More than likely though, I generally have something else to do after my delivery which precludes me from being available to stick around and cut cake.
Am I the only one who has angst about cutting my cakes? I just don't like to do it, all of my hard work decorating, destroyed with one single knife swipe. I will do it, but I'd prefer someone else have the honor.