To THIS question, I can speak with experience. In my car -- exact symptoms you have -- I changed it with NO difference. I also unplugged the idle valve for a test and had no difference. My plugs were changed 5,000k ago, and no difference.
1994 C280, dark green metallic
1991 190E 2.6, Black
(sold to another forum member)
2003 Chevy Tahoe LT, Redfire Metallic
1989 Mustang GT Cobra Convertible