How do I, as a consumer, gain or lose when the utility company line voltage varies from rated value?
The meter has a voltage coil, or measures voltage in some way, so you are only paying for the energy that passes the meter. This much is fair.

What happens next depends on your equipment, but it doesn't make much difference.

If you run predominantly switch-mode devices like computers, inverter-driven motors, LED lights, then they will take the same amount of power at any voltage. At a higher line voltage, the current will drop, and hence the power wasted in the wiring will decrease. This is probably only a 1% effect.

Resistive loads with thermostats like a hot water cylinder will show no net change - both the power of the element, and the power of wasted in the wires will go up, in proportion.

If you have a lot of light bulbs... your power consumption will go up at higher voltage. While you will get the extra light that you're paying for, you might not have wanted it. If you already have enough lights, 10% more light for 10% more money is just a waste.

There are probably other cases I haven't thought of. Induction motors... transformers.