The "memory effect" is not typically a problem with modern batteries. The effect is generally "broken through" by large discharges, too, so if it's a battery for a power-tool, then it's a self-correcting problem.
Modern chargers won't over charge so you shouldn't have a problem. Power consumption for continuous "trickle charge" is tiny. Opening your refrigerator an extra time will probably amount to a year's worth of such charging.
Battery longevity may be reduced by it remaining warm for an extended period but it also degrades just sitting around.
Bottom line: If it'll be a problem for you when you need to use your electric whatever and there is no charged battery, then leave a battery in the charger.
http://batteryuniversity.com/ has some of the best information about batteries. I'm especially fond of this article on the maintenance of Li-Ion batteries: http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries