Process best practice - when to use Purchase Orders or use for everything?

We're just starting to configure Netsuite and looking at our business processes and wondering if we should use purchase orders for absolutely everything, and if we do how we get around certain problems.

There is an argument that POs should be created for everything so that the cost is recognised, however there is also an argument that some items you can't 'choose' to purchase - such as electricity, gas etc... - and does creating a PO really add any benefit?

Can we use blanket orders for this?  especially as we don't know what the cost of each invoice would be?  or should we not create a PO and instead process as a bill / invoice with no PO?  if we do create a PO, presumably we'd have to do the unit cost as £1 and the quantity at say 10,000 for a one year order, then 'receipt' and process the invoice by quantity, ie; a £126 electricity invoice we'd received and process as a quantity of 126 to get the ride figure, otherwise how will it match if we can't