I was just reading an article about something totally unrelated, but they mentioned the old saying "don't burn your bridges" and it got me thinking. I was just teaching in my HF course about decision making and how we think that having more options makes for better decisions, but it actually makes us worse. The cognitive heuristic of loss aversion makes us fear making a mistake in choosing one option over another. The more options, the more fear and the harder it is to make decisions. Not only does it take longer and require more cognitive resources, but the negative emotion also fundamentally changes the way the decision is made - forcing attention to small details and losing the big picture.
So my thought was that it is often good to burn your bridges when it reduces the number of options we have to make important decisions. Whether you are choosing a major, a job, a relationship, or even what to order from a restaurant menu. By eliminating some choices from the consideration set, you make the decision faster, easier, and more reliable.
So if you leave an interview thinking you don't really want the job, tell them "no" as soon as possible. Don't leave it on the table thinking "well, maybe it would be a fallback position" or "maybe I will change my mind later." I realize with the current job market, this may be difficult and a bad job is better than no job at all when you have mouths to feed. But in general, I think it is a good approach - at least a better approach than we think.