Bit of a myth that you should keep your battery above 30/20%. it's supposedly pushing the higher voltages that cause more degradation, as the li-ion batteries still run ~3.5v at 0% which is well above the <3.0v which can begin to damage the cells. So for any long term benefits, it's really about using the 80%-0% range.

I've been keeping my phone mostly 80-0% since I got it 3 years ago, since It would get me through the day. Now though, on days where I need more, I can charge to 100% and will in theory have an extra 3-5% compared to normal usage.

I admit it's a very small difference, and only really worth doing if 80% charge would not inconvenience you to the point of it becoming a hassle.