You can go either way. It just depends whether you need the additional visibility on flows that might be confined to lower layers of the topology. NetFlow is interface-specific, so you need to export from all interfaces for which you need detailed visibility. If you only need visibility at the L3 point in the topology, that will be fine. However, be aware that if you do export from multiple points in the network and the same traffic crosses multiple interfaces, you'll get double-counting in your reports if you run reports that aggregate across those different interfaces.