I would skip them. They don't actually gain you any power, they just divert some of the power that normally is used to drive the accessories like your alternator and such back to the wheels.

On some cars where extended high-RPM running tends to mess up the alternator, an underdrive pulley can be useful since it spins the alternator at a lower speed for the same given engine RPM as the stock pulley. In most Hondas that's not really an issue so there's not much of a reason to use one.