This came home to me in a big way when my weed guy came around to spray. He was filling his tank with water from my hose and mixing in the various chemicals he uses when the power died. The pump shut off and the back flow prevention device on the hose he was using did what it was designed to do; dumped water on the ground that might have made it back into the house water. Now, there was no real danger because he had the fill device on his tank set up properly to prevent back flow and I had a vacuum break (back flow preventer) at the end of the hose, however it does illustrate that something can happen when you least expect it. There's been many times I was filling the pool after doing something that the power could have failed and caused a siphon effect to suck water into the house supply. So, you need these things.
Where I live you have to have a functioning anti-siphon device on all hose bibs before you can pass inspection to move in. However, almost everyone uses one of these:
So, what to do? There are a number of possibilities including a vacuum break for the whole house, but each of the solutions has a drawback that I hate. For example, the whole house idea will work, but I'll have to drain my house plumbing if there happened to be a problem. I think I have found a solution to this. Arrowhead Brass makes a nice anti-siphon hose bib that isn't too expensive and seems to do the job just fine. Their web page here shows the different styles, and for you folks that live in cold country they even have the no-freeze designs. I bought three of these: