Sim culling happens when the game reaches the max number of sims allowed. What I do to prevent a sim from being culled, and not allowing the game to decide, is I will make the number of sims allow set up unlimited. This way I can choose who I want to go or to stay. Regardless if you have that set to that or not, what you do to protect certain households from being culled (or deleted) is to click the little heart (make it a solid color). You can see this in Manage Worlds.
I read somewhere that the game allows up to 800 sims (as of Island Living) before culling kicks in. That also includes sims in the hidden household bin. This is something you can not see, unless you have the MCCC mod. Hidden household would also be animals, ghosts, and other service sims.