I ask these questions: Will there be another dessert? Is there going to be an open bar? Are your guests big cake eaters or more of a drinking crowd? Are you doing a candy or cookie table? Based on that kind of thing I recommend 80% plus or minus some of the number of guests for the number of cake servings, Not everyone has cake after a full meal, drinking and dancing, and there's absolutely no reason to provide 100% of the number of guests for the number of cake servings unless the cake is the only thing people will be eating. If they want to order more than what I'm suggesting that's fine, but I tell them that they'll probably have cake left over, which for some people is good, but some would prefer not to pay for something they're not going to eat.
There are also some kinds of weddings where there's so much food there's no way that people are all going to be eating cake. Indian weddings come to mind, but I ask about the food too in most cases. The venue managers who I talk to say that when the brides order a serving per guest they always end up with half the bottom tier left over. I can't see running out of cake unless you only buy half the number of serving compared to guests, which would be too low anyway. Most venues cut the cake smaller than we think they do, so you end up getting more servings out of the cake, too.