Hey Jimmy, hey Derek @WaywardWoodworker,
Jimmy, I think the accessibility of the emergency stop button is important however noone forces you to have your estop button on the controller case. Usually it is positioned externally in a little box so that you can best reach it during machine operation.
You should also know that the big red button on the buildbotics-based TOS Onefinity contoller is not connected in a way that it triggers the estopped mode of the controller! (at least not the last time I had a look). As Onefinity confirmed here, it just cuts the power from the Raspi and AVR Controller board, thereby also the stepper motors, but leaves the router and spindle running! Because the electromagnetic detent on all steppers is gone then, hitting that red button can cause the Z axis to move downwards with gravity, and with running router or spindle, can make a nice hole into your table top (or into your hand if it happens to be in between). Therefore when making use of an easily accessible emergency stop button in its own small external box, it is better to wire it to the “estop” pin 23 of the 25-pin I/O port of the controller to benefit of the buildbotics-based Onefinity Controller’s Estopped mode which ensures that the router (via relay on pin 15) and the spindle (via STOP command) are also stopped automatically when the red button is pressed. See here for details and links.