I don't think it is necessary

Remember, 485 is "adjustment of status", so that means your status is changed from whatever I-94 says to GC, so after you got your GC, your old I-94 is invalid anyway, so what's the point of returning it? Even worse, if you give it back, technically you are submitting an invalid document to the federal government, and that is punishable.
When you change your status for whatever purpose, uscis need your old I94 to link your new status to the old one. Otherwise, for example, if you switch from F1 to H1, do you need to give back F1 I94 when leaving?