Great question.
When I buy a new pack I evict all the premades that does not interest me and let them be 'filling' in the world, and used by the game in whatever way needed, placed in the Other Households tab.
My created sims, and the ones of the premades that I want to play, I decide all things for, choosing and changing any career choice not to my liking. I only need to set up one sim per career path (there can be more of course but I am not that far), and the rest I register with the Employment Office, giving them a proper job title and description according to their occupation, thoughtfully decided by me to fit my world.
I plan to fill the world with sims suitable for the service jobs as well.