After each washing you'll notice the twists will feel looser. Some people completely re-twist their dreads after each washing. It is not usually necessary to re-twist after every washing. Strong healthy hair (take your vitamins , eat as many raw fruits and veggies as possible) can withstand more re-twisting but if it doesn't need to be done there is no reason to spend the time to do it.
Re-twisting after every other washing is usually fine, especially if you are able to wash them carefully and not disturb the twists. One tip is to wash them while wearing a nylon stocking cap. This helps to protect the dreadlocks during washing. If you try this method take care to rinse the dreadlocks extra well since the stocking will tend to make getting all of the soap out of the dread more difficult.