I don't think it really matters too much which direction the water flows, except in the case of a temperature control valve, which should be first.
But if you didn't have a temperature control valve, then the main concern would be routing to purge air out of the system. That diagram shows the connections at the bottom of the heater coil, which would tend to trap air in the top of the coil, but maybe there's enough flow that air trapping isn't a problem.
