How important is it that wood siding be installed over strapping to provide ventilation?

It’s always possible for any kind of siding to leak water, and that’s why some kind of back-of-siding drainage pathway is a great idea. Installing siding on strapping is one option, and there are plastic and mesh-type products made especially for the job. I’m getting ready to install cedar shingle siding on a project of my own right now, and I’ll be putting the shingles over a dimpled plastic drainage sheet called DELTA-DRY (888-4DELTA4). Any of these options are a sort of “plan B”, just in case of leakage. That said, most wood siding installations in Canada don’t have effective back-of-siding drainage and it’s not the end of the world. If I were the prospective buyer of your house, I wouldn’t let lack of strapping stop my purchase. Quick paint deterioration is often an indication of leakage problems, and it’s a good sign that your siding is holding up well.