Ultimately, you are unlikely to ever encounter the problem, in all honesty. But the supressions diodes are the way to go in the 1 out of 10000 (just made that number up, btw) chance that it will happen. I work in the aerospace industry where the fuselage of aircrafts are obviously much more susceptable to high voltage spikes (both from static builing up and from electrical storms) and this is what we use on every signal that goes in or out of our electronics. I have seen them returned blown quite violently, yet the circuits themselves were protected.
Now, of course, a house is NOT an aircraft. You are not putting your house in such an environment and your house does have an earth ground (well, it better have) where the aircraft does not.
So, I guess the question is how anal do you want to be about it? Do you want to protect against all possibilities, or just take your chances (which are pretty good)?
That's the trouble with asking engineers this kind of stuff. lol. There is the right way, the wrong way, and the engineer way. The engineer is used to having to deal with standards and regulations like UL and CE, etc... which covers all kinds of whatif type scenarios. When we answer your questions, we are likely to make things sound like a scary world is out there. But those what-ifs DO happen on ocassion and the standards are the result of decades of experience and seeing the results of the what-ifs.
My only real motivation for pushing you to use supression diodes is that this will be permanently installed in your home. Being permanently connected increases your chances of encountering a what-if scenario. The whatif scenario may result in a fire. In your home. Where you sleep and where those precious to you sleep. That's why we have UL and CE standards.
A MOV or varistor is very cheap method:http://en.wikipedia.org/wiki/Varistorhttp://en.wikipedia.org/wiki/Surge_protector
Whether you want to use them or not is up to you. I guess I just can't really understand why you wouldn't want to. We're not talking about having invalid and false alarm signals from your sensors. We are talking about fires. The first time one blows and you look at the result of it, you will be glad that you did.