When the battery is left in the laptop when the ac is in the laptop will draw a bit from the battery and most from the ac...but as you draw a bit the laptop starts to charge the battery. IT continues this process. So if you had the laptop off when charging there would be nothing being drawn except for the tiny bit that leaks
The ammount drawn from the battery is ,if anything, extremely insignificant and therefore it doesn't really matter if you have the battery in and charge at the same time.
And as Calc said, the new Li-Ion batteries aren't affected by being charged/depleted before full charge/depletion has taken place.