Wiring home and limit switches

[JAZZCNC]

When switching off the 70v supply should it be switch before or after the transformer or does this not matter?

It's arguable but you could break circuit after transformer but me personally for safety sake then I want power killed. Same goes with VFD.? you could just send a stop signal to vfd but again for safety sake just kill power to vfd.
It's like with the drives you could just kill the enable signal and keep the drives powered but that's not safe enough for me and I want the machine off line full stop.
This is why you have a master relay.? One relay controls all other relays and if the other relays which are controlled by limits, E-stop,Door,coolant switches etc are not safe or ready then master relay won't allow a reset.

Do many people have an input to the 24v system from the pc incase the computer crashes / turns off for some reason acting as an e-stop or would the code just stop running the anyway not harming the machine?

Well that's the job of a charge pump feature if BOB has it, usually from pin1. Charge pump monitors pin 1 and if it doesn't see a signal shuts down it's outputs etc. Not full safety but enough to E-stop the system if PC crashes which then if designed correctly should shut down the master relay which kills power to every thing else and not allow restart.

So that the drain resistor isn't constantly connected during the 70v supply is it connected via a relay so that when power is removed the resistor is engaged to drain the motor supply system?

Yes exactly just use a NC contact so when working it's disengaged from circuit and put back in to drain caps when off.