It doesn't matter how your city looks right now. At first you should always target the 30% probability to bring new citizens into your city. The easiest and also the cheapest way how to do it is by buying a lot of Park cards, but in the long run the Art Gallery cards are also very good.
When you already have the 30% probability it really does make sense to concentrate on Job Center cards. These cards will not only increase your SIM income a little bit, but will also train your new homeless and immigrants cards into real workers, scientists and artists.
You can see in the picture below that every day I mine at least one new card and with the help of the Job Center cards some of the homeless and immigrants cards are trained rather quickly.
Even if I would stop playing the game right now my city would still be getting bigger and bigger and that's the most important part. It's an idle game so in my opinion this is how it should be played.
Good luck!