We use add-on programs from ISTECH.
 
We enter a top level item and then generate multi-assembly WO's for our production.  
 
We then use the "Convert WO to PO" program to automatically issue materials already on hand and to automatically generate PO's for the items that we do not have on hand.  Each item that is issued or ordered is assigned to the WO on which it is used.
 
"Print consolidated PO" program also consolidates multiple items needed from a vendor into one quantity before it orders.
 
We really like these programs.