jm977: The absolute accuracy of the cameras battery reading isn't that good. If it dropped from 100% to 90% in a day, I would be concerned. 100% to 99% is well within measurement error.
Garfield - Thank you very much for that data!
Based on Garfield's data, the "OFF" power drain is little more than the self discharge rate for the Li batteries. So even with the battery sitting in your camera bag by itself, it's going to discharge eventually. Unless you plan on putting the camera away for a year, I wouldn't worry about it.
Personally, I charge both my batteries before a shoot, and when things are quiet (usually meaning the day job is too busy to enjoy much photography, although there is always a photo or two during a normal day with my cats...), I swap the battery in the camera once a month or so (charging the one that has been in the camera). I've not found the in camera battery down more than 20% or so.