Morning is a useful default
Early watering gives roots access to moisture before heat builds, while leaves have time to dry. It is a simple routine for many outdoor plants.
Evening can be better than skipping
If a plant is clearly dry and morning is many hours away, water it. Avoid turning a good rule into a reason to leave a plant stressed.
Water the root zone
Aim at the compost or soil, not the foliage. A slow soak at the base is usually more useful than a quick spray over the whole plant.