Hi Sam,
I built the tub and fitted everything as it was completed (axles, windscreen etc) just to test fit and get the holes on the right places.
Once this was done, I stripped the parts off and painted the tub in one lot. This way, I could paint inside any holes so that moisture couldn't get into the wood.
It also gives a 'fully finished' and neater appearance to the car, no splashes on wiring or other bits left on.
I suppose it's up to you really, but you want the car to look 'professionally' built, rather than 'bodged'.
Alternatively, you can fit parts and get holes in the right position, then remove them until the tub is painted, and then assemble the vehicle.
Regards,
Mark.