Don't do that. :)
The items under the folder don't EXPLICITLY inherit the importance/urgency level of the parent but when those task items have their "score" calculated (the score determines how high up on a task view (non-outline view) a task appears) the importance/urgency of the parent is already weighed and considered. So for example if you touch nothing but go to the top level parent branch and crank up importance to the max, that branch and any/every task under it will rise in importance relative to all the other branches.
Another way to think of it is that the import/urgen scores are relative to other tasks at the same "level" and/or "sub levels" but with consideration for the importance/urgency of all it's parents as well.
hth.