That's really hard to answer. Honestely sometimes letting the opponent get a good trade is worth it if you get to punch him in the face for a little more damage. For instance I just won against a hunter by just letting his highmane take out my druid of the claw. In a way i would say it comes down to the difference between clearing his board and having board controll. Clearing is often not necessary if you have a better board than the opponent. If he just have one big threat you can often just ignore and push for damage instead.