Jason Cohen proposes an interesting idea for customers asking for a discount: give them two months free if they prepay for a year. He makes the case that if you're a bootstrapper having the money all at once upfront is worth it.
But even if there is a liability you have to calculate the percentage of people that will want a refund. If it is "high" (whatever that is..) you have a much bigger problem (people don't like your product or service).