It's all personal preference. Mount everything where it allows you to work on the car the easiest, route the wires the easiest and be the most functional for you. It doesn't matter if you mount the ESC upside down on the shock tower, performance will be the same.
Typically, I mount my ESC tabs and the side of the Receiver that allows you to plug the components into it toward the outside of the car. This makes routing wires and soldering an easier job. Plus if you route everything in towards the drive train of the car it's just one more thing that has a possibility of getting caught in a belt or shaft. I move wires as far away as I can from the drivetrain for that very reason.