I've read that charging all the way down might be suboptimal. The recommendation to do full decharge and then recharge is for older battery technology from many years ago. But that recommendation still lives on in lore.
Reality is that you shouldn't worry about it. iPhone 5 is supposedly most repairable iPhone yet. Battery replacement a year or two from now should be fairly inexpensive or something you can do yourself in 20 minutes for $20 in tools and a $10 battery.
Quote from iFixit:
In the 3GS, the display assembly was easy to take out, but the battery was a pain. In the 4 and 4S, the display assembly took 38 steps to remove, while the battery was a breeze. We like to think that Apple has started tailoring to our preferences in allowing us to remove both the display assembly and the battery in only a couple of steps.