It really depends on the person. I erectors have grown just fine, so I've never needed to hit them directly. Again, they are the slowest recovering muscles in the body (AFAIK). So many exercises work them that, for me at least, I don't have the time needed to recuperate from a dedicated effort to work them out.
Squats and Deads will work your erectors ever so painfully all by themselves.
Again, only you, or your trainer, really know if you need to work them directly. If you are very new to bodybuilding you may need to. Most out-of-shape people have a very weak trunk. So much of the physical things you do in life (lifting, pulling, etc.) require the stabilization that your trunk provides. This is why so many people believe that the back muscles are so fragile. If you new to bodybuilding, take a little extra care in building up those muscles (erector, abs, obliques) and you should do fine.