I don't think there are any absolutes across hundreds of stores, but if you made a reservation and show up at your time there should be no problem. And, why wouldn't you expect the Apple store to, in effect, forfeit your reserve if you don't show or leave because "Not everyone can waste an hour+ in the apple store in release day..."? An airline will give your seat to a standby passenger if you don't show up.
airline is different, you are purchasing a seat to travel on X date. The apple product can exist on any day. So technically its not a reservation but a "right to purchase on Friday"