we only prorate on move-out - we are an anniversary billing company. they must give 5 days notice, and we prorate only 15 days into their month. after that, it's all or nothing. if they don't give 5 days notice, they get charged a 5 day rent fee. and once they prepay, the don't give any refunds. we only refund a full months rent. it seems to work for us.