Known icing conditions are just that. If you fly and collect ice, that is known icing conditions. Some days the forecast can be incorrect, or just not accurate enough. A half a degree C can make the difference between collecting ice or not. You could change your track by a half a mile and be in (or out) of icing.
If the forecast indicates icing may take place, that should be used as a (strongly advisable) guide.
BTW, the engine has anti-icing protection, not de-icing. Anti-icing prevents ice formation. By definition, de-icing insinuates that you have already flown in icing and collected some ice.