Once a month they (Apple) recommend you fully drain your battery and then a full charge before fully draining and a full charge again. This is suppose to make your battery last longer. It seems to last longer when I charge it this way. Hope this helps.
Your iPhone is like a laptop, you don't want it to die completely if you can help it. I usually charge mine soon after the 10% warning pops up. If it gets to the 5% range I usually turn it off if charging it isn't an option.
Batteries have memories. The more you charge them when it is unnecessary then the faster they drain. It is ok to let it completely die at least once a month to retrain the batteries life memory. If you call Apple support they will give you more detailed 411