The Great Career Imbalance

I keep a balance among my played sims, and when a premade marries into a family I'll change their career to keep the balance, but I don't mess with them otherwise.

I change base game sims to jobs I think they would have (following lore) and make sure their skills match up.

I dont touch DLC sims much... maybe match their skills up to their pre-existing careers.

I don't care what pre-mades and townies do, unless they somehow make it into my rotation/families. My own sims have had many jobs, I like to try them all at least once.

I don't often change NPC jobs because I don't usually want to take the time to do it, but I'm in the process of setting up a personal Save File that I'll use for future saves (once it's set up) and I plan to make a lot of changes in that, both to jobs and relationships.

Sometimes I make them work from home, go to college, or just not work at all. Nice to have them run a business sometimes too.

I do change some sims careers but not a bunch of them. The ones that I do change, is to fill careers for story telling purposes. When I'm playing in general I dont care what the NPC careers are.

I never really bother with the premades, they end up as bartenders or vendors most of the time anyway.
But, in the save I'm building, I try to have an even spread between all jobs (for housed sims) and balance the male-female ratio as well. I'll have a bunch of homeless sims take on the NPC jobs once I start setting things up (and add some mods that help with that).

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.

I don't do anything because it would make perfect sense that some Sims are working less lucrative jobs. It adds to the world.

I only keep a note on my played Sims. Once they become played, I either randomise their career or I choose one depending on their aspiration.