Granpa,
Yes, you're right, the ESC doesn't really care where the input voltage is coming from as long as it's not too far out of it's designed input range.
Where it gets sticky is if the lipo gets discharged too low it can severely damage the battery...so more and more ESCs are becoming equiped with Lipo Saftey Cut Offs...which just prevent the battery from being discharged too low by cutting off power to the motor.
If your ESC doesn't have this built in and you want the safety of a cut off we offer a number of Smart Stop cut off devices for 2, 3, or 4 cell lipo applications.