Plants are just like people, they each have their own preferences. Read the tags or labels that come with your containers to find out the preferences before watering your plants. Information on watering and other general care requirements are almost always on the labels that come with the plant.
The type of device you use will determine the best time for watering your plants. If you are going to use a drip system or soaker hose than evenings is the best time to water. If you are using a sprinkler system, water in the morning so the foliage dries before nightfall. Try to avoid watering your plants during the heat of the day because you will lose quite a bit to evaporation.
The type of soil that you use for your plants will also dictate how often you need to water. If the soil is sandy, your plants will dry out faster than if the soil has a lot of clay. You can always improve the condition of your soil be using manure, peat moss or humus.
When it comes to watering your plants, thorough but less frequent watering is the key. Thoroughly watering your plants will encourage the roots to grow deeper. As the roots grow deeper the roots will have a larger area to draw water from and limits the number of poor-performing surface roots.
Water annuals when the top few inches of soil begin to dry. Most perennials, shrubs and trees have roots that are in the top foot or two of soil, so concentrate your water to those areas. For the first year or two, water perennials often so they can establish a strong root system. Once the root system has been established, you can water less frequently.