That's correct. For most people who got GC through adjustment of status, the "resident since" date should be the date of the I-485 approval. There are some exceptions, e.g. for those who applied for adjustment of status based on their asylum status, the "resident since" date is backdated one year earlier than the I-485 approval date. For those who obtained GC through consular processing, the "resident since" date should be the date of initial entry into the U.S. on an immigrant visa. As jackolantern notes, sometimes the "resident since" date printed on the physical GC is incorrect because of various USCIS screw-ups. So one does need to exercise common sense in double-checking the "resident since" date.