Tighten unused screws on receptacle?

Just saw a picture in another thread where the unused screws on a duplex receptacle are left un-tightened. I see this often on pictures posted here and have found it in my own home. Is there a code issue one way or the other? The old timer I learned from insisted on all the unused screws be tightened down. I have always made it a practice to tighten them. I certainly am not a code maven but have never seen a reference either way.