create PO without material master

Hello guru

When we should create PO without material , no dout materail group willbe assigned to valluation class , then accounting document is generated ,so my case is when u do the PO without material is it necessary to do GR, in this position any movement type is hitting are not iam confused plz clarify problem

(without material creat Po means is it right na to assign mat group to val class imgmm-purchasingmaster data---entry aids for items without material master)

here GR is possible are not plz give me clear picture on this)

Regards

sapman