Really? I've heard that by doing that what you are doing is reducing the time the battery will be charged (it's like instead of using 100% of your battery, it set its limit at 50%). I don't have the source where I saw this, sorry.
You may be thinking of the "memory effect" of old NiCad batteries, which indeed needed to be thoroughly discharged before recharging otherwise they'd lose capacity. However, LiIon is a different chemistry, and different rules apply. I don't know of any modern device that still uses NiCad, though I'm sure there's still some legacy niche where they're useful...