For stories yes and no. If the Sim in my imagination would be especially skilled at something, I give it to them. I play a lot of large households and micromanaging everyone to relearn every skill each time I start a new game with that household is just too much busy work.
Growing with a Sim, though? No need then. It just depends on what kind of game I feel like playing that day, and there is more to it than just grinding up the skills. (Much of my Sims 3 play would beg to differ, but I would be too busy grinding out the skill challenges on my fav sims to pay much attention to the rest of the household. Now I have time to actually kind of care about the characters I do not micromanage so much.)