When should I plug in my MacBook Pro?
You are right lots of myths.

and  more info this "kb/ht1446"

Apple Portables: Calibrating your computer's battery for best performance

The battery has an internal microprocessor that provides an estimate of the amount of energy in the battery as it charges and discharges. The battery needs to be recalibrated from time to time to keep the onscreen battery time and percent display accurate and to keep the battery operating at maximum efficiency. You should perform this procedure when you first use your computer and then every few months after that. If you normally leave your Apple portable computer connected to AC power and very rarely use it on battery power you may want to perform this process once a month.


Portables with built-in batteries (aka MacBook Air)


Current Apple portable computer batteries are pre-calibrated and do not require the calibration procedure outlined in this article.

In summary:

The battery has no memory effect &amp; can be charged whenever you like. You can't overcharge it, &amp; charging automatically stops when your battery is full.
The two biggest killers of Lithium batteries are heat &amp; frequent full discharge/recharge cycles. 
For calibration reasons (to make your battery status indicator accurate) you can discard but not less than 20% once in a few months.

Best way to prolong the life is to keep it as cool as possible. That might mean do not take your lap top in your bed, do not keep it in the sun, ect.