Unless you drastically increase the capacity of the HV battery, you won't save much by plugging it in.
You might think of that battery as a capacitor, not a battery. It's intended as a temporary reservior to store SMALL amounts of energy during stop/start cycles (or slow-n-go cycles). It only has one three-hundredth the useful energy capacity of the gas tank, so you just aren't doing much by topping it off overnight.
It's value is in repeatedly storing little bits of energy that would otherwise be lost. Normal driving, with it's ups and downs, braking and acceleration, provides many, many such cycles. They add up over time under these circumstances, but just one more cycle saved from an overnight charge won't even be detectable in your mileage.
The only case where this matters is with short trips (no more than a mile), and then you should use a bicycle!