I'm assuming by "approved lines", you mean the same thing as "approved requisitions" and not the line items literally.
In my experience, my customers, in general, had/have individual buyers to place the (order) requisitions and as well as take responsibility of "processing" them. So, the individual buyers get notified, by default, for approved requisitions.
However, for some special criteria, for approved requisitions, we use the second model mentioned by you. In these cases, the one who submitted the requisition will be different from the buyer to which the approved requisition would be routed. The buyer will get only the requisitions to be processed by him.
Hope I understood your question and hope this information helps.
-Shashi