You can do this, but then you would have to include/select the BOM in question as a "Sub BOM" on your order (Production or Sales order).
The BOM MUST be ACTIVATED if Master plan is to take it in to account if this field is not filled out.
The reason for this is that you can have multiple BOMs approved for an item. It can be different versions or variations. But only one is "standard" and is approved.
So always approve AND activate if you want your BOM to be calculated (unless you plan on specifying this every time you create an order).