Aeration makes a big difference in the health of your fish because they're never starved for oxygen, which can occur in un-aerated ponds especially in the last few hours before daybreak.
But if your pond isn't overly deep, and you keep the total biomass well below the maximum carrying capacity of the pond, you can get by without it. Aerators specifically made for fishing ponds, i.e. ones that aerate the bottom water and not just the surface, are far better than fountains that just aerate the surface layer, as the former can prevent a fish kill if the pond turns over.
Aeration is definitely a good thing to do when you're fertilizing. Though your fish will grow much faster due to the increase in plankton, said plankton, if they're too numerous (if you can't see more than 6" down into the water two weeks after fertilizing, you've got too much bloom), can cause a fish kill if several cloudy days occur concurrently such that a large-scale plankton die-off occurs. But a good aerator prevents this, and thus is worth its weight in gold.
