The batteries have finite life, usually measured in cycles of use. Lithium Ion is not as susceptible to memory as the old NiCd/NiMh.
If it's not a pain, I would suggest removing the battery once charged and using on the mains without a battery, slip the battery in if you're using it without mains. If you usage pattern makes this easy, it will prolong battery life.
You should use the battery occasionally, at least once a month, and let it fully discharge then charge again, possibly twice, for conditioning.
Or (like me) you can just live with the fact that leaving it in all the time means it wears out quicker (holds less charge) and hope the laptop dies before the battery - my laptops rarely last much over 3 years, I now buy IBMs with a 3 year warranty, then buy another one. By the end the battery only lasts about an hour. Good incentive to buy a new one, give the old one to the kids!
Hope this helps.
BW