I have not heard of that. I know you can't use your Bluetooth headset when it is charging but not your phone. In fact I charge my BB everyday, doesn't matter if it is low batt or not.
When I just got my 7520 back then, I read from one of those welcome emails you get when you just activate your phone that it is adviced to charge your phone for at least 10-15 minutes a day. I have read that charging your phone when it is almost down the drain takes LONGER to charge than charging it a few minutes everyday.