There are a lot of misconceptions about charging cell phones, largely stemming imo from the older nickel-based batteries. The simplest way to put the matter is, with lithium ion, it's best to charge whenever possible. The higher the percentage, the better. Now, there is a bit more to that... Lithium ion batteries do not have memory issues like the older types do - if you charge it at 90%, or 20%, or even 01%, it does not affect the 'virtual' longevity of the battery. In addition, the lifespan of a lithium ion battery isn't significantly affected by what percentage you charge it at - they are rated for a certain amount of charge/discharge cycles (commonly 300-500, although this does vary), and charging from 90% to 100% is the same as charging from 00% to 10%. However, a full charge/discharge cycle does put more stress on the battery - you might have a small gain of lifespan if you charge with a smaller discharge depth. So you should be fine to let it drain, but if you're an OCD optimizer like I am, you're going to want to charge when you get the chance.
Drawing from this, if you do keep your phone on the charger, you don't have to really worry about it affecting your battery too much. The best case would be to eliminate draw (powering down phone, and thereby eliminating any need to recharge if the draw is higher than the charge), but this isn't really necessary - the difference is negligible. The major issue is extended periods of low/high battery charges - if you're going to leave your phone off for a month, try to keep it between 40-70% charge to minimize the degradation of capacity.
For more information, feel free to ask questions, or check out the pages I listed below.
Sources: I used to work in the cell phone industry.
Lithium-based Batteries Information How to Prolong Lithium-based Batteries - Battery University
(note: i also posted this in a comment in the moto x subreddit)
EDIT:
TL;DR: Charge whenever possible. The lower the depth of discharge the more longevity, but it doesn't make a big difference either way.
EDIT2:
The lifespan of the battery (I probably should have put this up there, will edit) is also not rated to a 0% capacity rating - this is to 70% of the original capacity.