In fact doing that may be harmful to your battery.

Modern Lithium Ion batteries like those used in Nokia phones such as our beloved N900 don't have memory and don't need to be primed or "trained" like old-fashioned Nickel or Lead-based batteries. It should work to its full potential right away and your N900's battery life during the first cycle should be no different than in the 10th or 20th cycle. You can charge whenever is convenient, no need to periodically discharge, trickle charge, etc. That's battery management of the previous era.

The "smart" chip on the battery tries to regulate voltage for charging and discharging, preventing the battery from discharging when voltage gets low. Sometimes this can be a problem when charging requires the battery to power on (like N900), if the battery is "too dead" it won't have enough power to go into the charging state, but the charger should still be able to s-l-o-w-l-y add charge to this battery until it finally has enough to enter low voltage mode and charge like normal. During this situation the N900 probably wouldn't even indicate that it is charging. So you should really avoid ever running out of battery completely if you can help it.

You should try to always keep the battery full whenever possible, or at leat keep it "not almost empty". Especially if you're turning off the phone overnight or removing the battery for a spare. (Charge the dead battery as soon as you can, don't leave it around for weeks.) Battery percentage remaining is really a guess, voltage is what it's really all about. The battery usually outputs a consitent amount of voltage until it gets close to dead and then it starts to weaken. That's why it may sometimes seem like your battery is on "full" all day and then suddenly declines rapidly. The decline is not linear. There is logic in smart battery systems to try to estimate based on experience that "X minutes at Y voltage means Z energy was used" but from one device to another this varies.

The smart chip also prevents it from overcharging, so there should be no harm in leaving the phone plugged into the charger even when the battery is full. It knows better than to overcharge it. (beware of generic aftermarket battery chargers that might not honor this "smart charging")

Personally, I leave my N900 plugged into charger whenever I'm sitting at my desk, or as soon as the battery indicator is not "full" I will put it on a charger my earliest convenience. I have chargers at work (USB), car, home (wall, USB) so I'm usually near a charging source.

If you have an extra battery you should keep it fully charged and store it in your refrigerator or other cool place. Warm temperature will reduce the effectiveness and lifetime of the battery.

It's also better to avoid rapid charging or discharging, and ideally the rate of charging should not exceed the maximum rate of discharging (but nobody really wants to wait all day for a charge, do they?). So using your USB charger is better than using the wall charger, since the wall charger pumps energy into the battery more quickly. It does of course take longer, but if you charge frequently there won't be as much charging needed, so time is not as important.

At least, that's AFAIK. IANABE (I am not a battery expert)