The idea that you need to "condition" a battery by fully discharging it, and then fully charging it, is an old wives' tale that dates from the days of NiCad batteries. Lithium Ion batteries don't need any such cycle, and you'll not find a battery manufacturer that recommends it. If it makes you feel better, though, don't do it more than once.
The two things that will shorten the lifetime of LI batteries more than anything else are overcharging and fully discharging. The phone's circuitry is smart enough not to overcharge the battery*, and I suspect it's smart enough to shut the phone off before the battery is really fully discharged, too. But why take a chance, since it won't help anything, anyway. Car manufacturers that use Li batteries keep the battery between 25% and 75% in order to maximize battery life. Of course, replacing the battery in your phone doesn't cost $8,000 - $20,000, so it's not quite as big a deal, but since there's no advantage to letting it drop real low there's no real reason to do so, except in an emergency.
* It IS possible to overcharge the battery in your phone a little bit, if you use the "bump charging" technique. This involves charging to 100%, removing from the charger, waiting a minute or two, then plugging it back in, and repeating multiple times. Each time you do it the battery will accept a little more charge, and get a little hotter, and lose a little lifespan. Just say no.
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