I know that my feet will flatten out by midday, and this seems true no matter if I've been running, hiking, or just tromping around doing chores. So I buy my shoes in the afternoon, and just get what's comfortable. I've never heard of anyone buying shoes at incrementally larger sizes depending on how many miles they plan on covering. Buy what's comfortable, and plan on readjusting your laces a few times a day during breaks. By the logic of your shoe salesman, you'd have to buy several pairs of the same shoe, then pick the pair of the day depending on how much you plan to hike/run. That'd be overkill.