When should i take pain relievers and when should i take a muscle relaxant for back pains?

Depends: Muscle relaxants and some pain medications (narcotics) can make a person drowsy or decrease reaction time. If you need to be alert (to drive or operate a machine), those medications should be avoided. Otherwise see which is more effective for you.

Non Steroidals: Taking non steroidal medications like Ibuprofen or Naproxen as indicated on the package, or by the prescription is a basis for pain control. Add a muscle relaxer when the muscles cramp, or are tight. Stretching exercises work the fastest to get yourself out of back pain.