I always unplug and let it get pretty much down to zero again. Doesn't that help the battery last longer?

Letting the charge run down is supposed to keep the battery operating at max capacity.

At least on my laptop (13" macbook), I find that if it reaches 100% and I leave it plugged in for a while, that 'fully charged' diminishes on its own. That is, after a day or so, it will read 99% even though it is fully charged and plugged in. Left another day, it will read 98%, still fully charged and plugged in.

I assume this means the battery is losing its capacity. So I unplug it and let the charge run down. When I get the warning that I'm running on reserve power (less than 5%), I plug it back in and charge it back up.

Apple laptop batteries have a lifespan of about 350 cycles. That means that if you let it run all the way down and then recharge every day, your battery will only last about a year.

I learned this from my husband's experience, when his battery died he called Apple and that's what they told him. Before that, I had also been under the impression that letting it run all the way down was the way to go.

When I worked for Apple, I found there was a lot of confusion and misinformation where Lithium-ion batteries are concerned. I found this article was a quick way to educate myself and others.

http://www.apple.com/batteries/

Following the instructions for calibrating your battery once a month is a good idea, but there is no need to drain your battery on each charge.
As for me, mine usually stays plugged in, but that's mostly because my battery is approaching the end of its useful life.