Just to clarify, 10% is OK too, but the "Low battery" flag is set at 15% so there's no benefit to draining any lower. The only reason why you might use it past 15% (into the bottom 15%) is if you have to make calls quickly, or need to end a browsing session, respond to emails, something pressing. Otherwise, you should stop use at or as close to but just under 15% as possible. 10% is fine and doing that once will have an immeasurable effect, but discharge below 15% on a regular basis and you WILL reduce the battery life.
See
charging my razr for a pretty in-depth discussion, but this graphic below says pretty much everything you need to know. Stay out of the "Zone of Temptation" if at all possible, run in "The zone of long battery life" instead and your battery will outlast the phone's expected "end of life".
Note: This chart is for a battery that has a rated capacity of 1,400mAh at 4.2V. In this example, they aren't charging it to 4.2V (100%), but instead to about 3.8V (closer to 80%) so it doesn't directly represent the remaining capacity of our batteries at the 15% mark, but the point where the curves begin to drop off (left edge of the red box) is the at about 20% mark for this battery, and so it applies to all batteries of this type, so just multiply any of those numbers across the bottom by 2.36 to see where our batteries will be. In this case, "The Zone of Temptation" starts at about 20%, but our phones have the low battery threshold flag set at 15%, so call that OUR bottom number, or where WE enter "The Zone of Temptation".
Good luck!
