Dark energy is density is constant and that's something like 75% of the universe, so I am pretty sure that the net change must be positive. But photons redshift and so loose energy. I assume other wave-like particles (all of them?) also redshift and so energy is lost. On the other hand matter (galaxies, dark matter etc) moves apart and so we have more gravitational potential energy and so more energy. So a proper accounting would be interesting.
Perhaps the answer is different at different cosmological epochs, since the energy change depends on the inflation rate.