It doesn't affect the battery but a full discharge at least once will make the battery meter more accurate.
I saw this on another web site: "The lifespan of your lithium-ion battery should last between 400-500 charge and discharge cycles. Depending on your usage, this usually amounts to 2-3 years. After that, your battery will have a reduced capacity charge and only hold 80% of a new cell. So in actual fact, your lithium-ion battery can last a good while longer than 3 years."
That site does not mention whether those cycles are full or partial. However, after the 400 to 500 charges one can still get an 80% charge and for our Paperwhite readers it will not really be that noticeable.
In addtion, most of us will be buying a new Kindle every year so, even if we charge every day, that would only be 365 charge cycles - well within the 100% capacity.
The referenced site is here:
I charge my PW every 15 days and get between 10 and 12 hours of specific reading cycles. I charge my KFHD7-32 every 12 days and get between 4.5 to 7 hours of specific use for app functions and reading.
I notice that the smaller day cycles are associated with the larger use hours and visa versa. Thus, you could get 20 hours of battery use on both if you used 8 hour reading sessions. Most of the time my reading sessions are much, much smaller.