I am also a newbie to vaping, but batteries, especially Lithium Ion ones, are one of those things I am always quite cautious about. The reputation of the company producing the product/battery usually gives you a good idea of how safe it would be.
The only time I have ever charged a battery overnight is when I bought a brand new ego ce4 kit, and in trying to charge it for 8 hours forgot to turn it off before I went to bed, and I had a mini-panic attack when I remembered that I left a battery charging overnight. I may be over reacting, probably because I have seen lithium ions explode in my place of work.
With ego type batteries especially, you probably won't know where it was made and how the QC is, the charger output may also be higher than it should be. On top of that, because it is something you would use all the time, you might get some moisture on the connections from either e-juice, wet hands, etc. This applies to all sorts of batteries, but I wouldn't use my laptop in the loo, or leave it on a table beside my beer =). I can already see how some might start to villianize e-cigs because of a few indidents!
I usually only charge my batteries in the car because I know I would be there if something happened, and if I have been using it a lot and it starts to get hot I would put it right in front of the a/c vents to cool it (I live in a very hot and very humid climate). BTW, rule of thumb, if any battery gets too hot to touch, you don't want to be near it, nor should it be near the power strip that you are probably charging it on... or your laptop which probably has a much larger li-on battery. =D
