Water hammers should be used on piping that has quick closing valves like solenoid valves or flug valves. Water hammer arrestors should also be used if you have higher pressures or velocities in your domestic water piping - or if you have a booster pump in operation. Common installation of plumbing piping in a chase is use of decreasing sized piping as the fixture units decrease (like 2"->1.5"->1"->0.5" as you go from water closet to lavatories. As you decreas in size, the velocity will go up. If this water is stopped suddenly, you get a pressure wave and water hammer happens. I typically use a full size manifold pipe in the chase - whatever size I brought into the chase. Then each fixture ties into that pipe. This decreases the chance for the higher velocities and the pressure wave. If you only have one or two fixtures - like a single toilet room, you could probably get away with no water hammer arrestor. But I would put it in anyway, just to be safe.

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