I paint as much of the finished model as possible but things like road wheels and drive wheels should be done off the model. Usually my sub-assemblies for painting are as follows;
the turrett, the hull, and the wheels.
I applied a trick seen in FSM a few months ago. A modeler had put bombs on an electric motor to paint the stripes on the front. I now use this method to paint the black outer edges of road wheels after I have airbrushed the center the desired color. It works great and it means not having to make circle shaped masks or having to perform careful handpainting.