I can't see how it would matter. Unlike wall warts and bricks plugged in at home, the USB and 12V outlets in the car should be dead when the car is shut down.
In many older cars, such as the 1989 my spouse uses as a daily driver, the 12V outlet remains live when the car is shut down. Things plugged in there will waste power. And the waste isn't tiny, it can be enough to drain the 12V over a night or weekend. USB had not yet been invented back then. With the enormous proliferation of battery-killing plug-in devices, the car makers have since had to turn off those outlets. All Prii were built in this later era, so are safe from this wastage.
