It would be very helpful if users had the option to automatically have field "Assigned User ID" on the Sales Order (and Purchase Order) automatically populated with whomever created that document.

This should not be a default for everybody, but there should be some setup on "Sales and Marketing Setup" (and "Purchase and Payables Setup"), to enable/disable this feature.

I have seen many implementations where customers request this modification, because they want to know who created which orders, in a better way than having to go to the Change Log entries.

This gives ownership of Orders to users, and customers like it.