Do NOT drain a lithium batter below 20% if you can avoid it. That will damage the battery and perhaps the BB as well. LI batteries seem to perform better if they are kept topped up when near a power supply. I always keep mine plugged in when I am in the office or car and have no problems at all.

The accuracy of the meter on the BB depends largely on the theme you are using. Some have incorporated the meter better than others. I have found the themes from RIM to be very accurate. The same with PocketDay.