This to me is the hardest rule to follow, mostly because I've never had a "real" running coach beside Coach D back in high school. When you coach yourself, you might look at number and think you're doing it right, but... We always think we can do more or work harder, and we overlook any soreness as just something we can push through.
Even with my experience of running for over 30 year, I want to ignore the little soreness and press on to improve. But the truth is rest is part of training. Taking one or two days off a week will not stop your improving, but it is almost a sure bet that you will injury yourself if you don't take some time off now and then.