This is one of those 'it depends' questions. Are you having detritus settling on your rocks/on your sand? Do you have 'dead' areas in your tank, areas where there is no/really low flow? Do you have corals that are not doing well (i.e. have algae growing on them or are closed up)? Have gasping fish?
If you answer yes to any of the questions above, then having an additional power head may not be a bad idea. Otherwise, I would wait and see how things are going in your tank.