Bottom-line: the batteries in laptops over the last decade are very different than the older batteries. You can leave them plugged in. There is no reason to run the battery all the way down at all anymore.
Most of the batteries have hundreds, if not a thousand charge cycles. With these Lithium-Ion batteries, a cycle is cumulative, in other words, if you run your battery down 25%, recharge it, run it down again by 25%, recharge it, and do it again twice more you’ve totaled 100% discharge. That total 100% discharge is one cycle.
These newer batteries will last many years and take “abuse” that older Nickel-based batteries could never survive.
From: Scott xxx
Subject: Laptop Battery
Craig, I know that you spoke on this a couple of months ago, but I missed most of it getting in and out of the car. Should (can) I leave my laptop plugged in or should I regularly cycle down the battery?