swapping out tires between each trip would get expensive. or are you talking about swapping wheels? seems like either one woud be a pain. do you need m/ts for where you are going? why not just swap on the stockers for the whole summer if you're going to be driving a lot on the road?
if you're just takling about gas mileage, do you know how big of a difference you are getting from m/t to stock tires? if its not much I wouldn't worry about it.