It may seem that any watering is better than none when it comes to plants, but if you want that watering to make a difference, water early, before the heat rises and so that the plants are not wet during the night, when fungal diseases can take hold.
There are many watering tip sites, but here is a good article and water conservation tips from the landcare association.
Some watering tips from the professional landcare network are:
•Water early in the morning when temperatures are cooler to minimize evaporation and reduce the opportunity for lawn disease to develop.
•Don’t water on windy days.
•Most established gardens do not need water more than twice a week.
•Water trees deeply. Use a soaker hose to apply water. You’ll have to water trees and shrubs less frequently if you soak them well.
•Maintain adequate fertility and proper soil pH.