It depends on how bad they are.
If the dentist can get to them easily, a local is usually used (unless anesthesia is requested).
If they are infected, inflamed, hard to get to, etc. you're usually knocked out. [it's easier that way]
I had to have mine done twice. The first one became infected (no idea how, I scrub the heck outta my mouth) but the others were fine. Dentist said he wouldn't touch me unless I was out due to the location of the tooth and the issues I was having. He wanted to do all of them at once, but I didn't have insurance and wasn't willing to fit that bill.
Fast forward 3 years... I have insurance now and the other bottom tooth had issues. (Tops ones no where close to coming down). Dentist's office said that if I got put out for 1 tooth, once the others came in, I couldn't be knocked out on their dime. I had the other 3 done all together. 1 was pulled, the others had to be cut out.