Most Lithium batteries can take 500 recharges before dying. Thus, it pays to let the battery run down to one bar before recharging.
I keep eReaders in shutdown mode when not in use to minimize discharge and, thus, minimize the number of recharges.
My Sony PRS 900 goes between 56 to 89 days between recharges using that doctrine. I expect my Kindle 3 to do the same.