Well.... I don't have a Dewalt charger, but I have two lithium Hitachi, a lithium Ryobi & an old 9v nicad Makita. The Hitachi's use 1.5 & 1.6 watts with no battery in them. The Ryobi dances... one second intervals, it goes from 0 to 2 to 3 to 5 watts, then back to zero. The Makita Nicad draws 5.6 watts.
I'd bet two donuts & a coffee that the Dewalt is similar....in my book, not enough to worry about. Also, if it is winter time, that 5 watts is just heat in the house! In the summer, yup, you'll have to AC that 5 watts back to zero.