It all depends on what strategy I have planned for the sim. In my Max Skills sim (I use that example a lot, don't I?) I need him to only get to level 10 of different careers just for the side effect of it. Becoming a level 10 science prof was a must have, as he will be able to improve objects inhome, which makes him pretty practicle in the family. Completing level 10 of the Fashion career is handy to, considering the permanent +20 moodlet he will reseive from it, but isn't a must.
You see, it al depends on the strategy.