Why wait? Stamp your passport ASAP for the temporary evidence of GC. You can not show the approval letter to your employer for work, plus you can not use the letter to re-enter the US. It is not the proof of your status. In addition, only after the stamping, BCIS will order you a physical GC. Learn a lesson from I-485, one month late in RD will result in several months or longer frustrating waiting time.