Should I leave NiCad rechargeable batteries charging when not in use for long periods?
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