text: You can check on http://oscompare.raggedred.net/?zoom=16&lat=55.90739&lon=-3.37077&layers=0B0TFFTF. A postcode in isolation is not mapped on OSM without a place with a full address. Most Satnavs will navigate to postcode centroids & people using them may inadvertently be adding notes with data from a non-open source. To re-iterate a single postcode value is no use for OSM.
  
  The postcode centroid data are already available (ONS data, CodePoint Open IS NOT SUITABLE FOR OSM) and AFAIK is incorporated in Nominatim searches. Such data can also be readily combined with OSM-sourced data in apps w/o needing to be added directly to OSM.