What about exit fees?
In the bad old days, energy plans often had fixed duration, and there would often be fees for early contract exit. Today there is regulation in pace that limits retailers ability to charge exit fees, so much so that none of them even try to charge exit fees any more.
Retailers do list 'disconnection' and 'reconnection' fees. These fees apply only to physical disconnection or reconnection of power from the property, which might be required if you're moving or for construction work. When you're switching retailers at the same address, your power is not disconnected at all, and there is no disconnect or reconnect fee whatsoever.
So you can use Bill Hero with confidence that your savings will not be eroded through exit fees.