This applies to daily watering, but I'm thinking of times during the year. We had a wet spring and I'm noticing perennials and shrubs are more vigorous than ever. Now, I knew that watering during April, or even March, dry spells was a good idea, but this year convinced me.
Doesn't seem like the ground's dry in April, doesn't seem like anything needs watering, but look at all those leaves sprouting up, the tall shoots lengthening, the shrubs beginning to bud and grow. This is the time they need the most water.
August, even July, is best for letting everything that's well-established (solid root systems) start to dry out. September? Forget watering at all. Not only are you saving water, but you're helping trees, shrubs and everything else get ready for winter. They'll be hardier and less prone to injury from freezes if they have a good solid dry period ahead of time.