It's not likely that the pipes would freeze buried 24" deep, but where they come up for air, they can and will. I would not risk leaving water in the lines even in the relatively mild Seattle climate. Just turning the water off in the house as you mention will be a good beginning, but this will still leave water in the pipes unless you drain the lines. If the lines are sloped back into the house, you can put a tee in the lines immediately after the shut off valve and install another valve in that tee as a drain valve. This would valve would be opened after the water supply was shut off, and the water in the pipe then would drain back to the house. You could use a hose, but a 5 gallon bucket would work to catch this water. This is simple enough that if you could use the water once in awhile during the winter. If it would be difficult or impossible to slope the lines back to the house, a small air compressor could be used to blow the lines out.