I would prefer to show a receipt rather than carry a mug that won't be used. It takes a lot of room in a backpack to carry mugs for a family of 4. We do as OP and get a soda on the way to Epcot or sometimes the bus stop and get a refill on the way back. It is silly to carry all the mugs when they are not used.
The only problem would be if a couple bought one mug and one showed the receipt for refills and the other the mug--am I the only one to foresee this?