Yes, it is important to drain you water heater once every year. If you are a homeowner, it is one more chore that you have to do, but it will make your water heater last a lot longer. This is because draining your water heater cleans out the sediment that collects in the bottom of the tank and also makes your water heater run more efficiently. All water heater manufacturers recommend that you flush the sediment from your water heater at least once a year. This should usually be done by a local plumber, and the cost of the visit pays off in the long run. As well, this is not a job you want to do on your own. There is not only a chance of scalding, but there is also a big mess to clean up if you don't run the hoses right.
Many homeowners don't understand what sediment is. Sediment is actually sand or grit that has gotten into the local water supply system. This is especially a concern around the Peninsula and Bay Area. The water pipes in the area can be extremely old, depending on where you live. Some pipes that transport water date back to the 1940's and simply have never been replaced. This means that sediment from these pipes come into your home every time that you use water.
How often that you need to drain your water heater also depends on where you live. The quality of your water may have much more of an impact on your water heater than you might think. Besides having old pipes in your neighborhood, if you have water with a high mineral content, then you may have to flush the sediment from your water heater more often than other homeowners. Call your local plumber or local water authority. They can tell you the amount of minerals that you may have in your water and how often you should drain your water heater to make it last much longer than what the manufacturer guarantees that it will last.