Anytime you can isolate a fixture or section of the house it is a good thing. Remember, if you close a valve everything down stream of it shuts down and some fixtures upstream just won't work because the water is no longer flowing, so you can over kill on shutoff valves. The bigger the house the more convienent it becomes when draining down a line. I have had water lines that have taken 45 min. or longer for the water to stop running. Fixtures that are replaced or areas of a house that are serviced on a regular basis should have at least 1.