To add to Brendan's reply, I think I'm right in saying that corrections will be shown back to 1/1/2000 only if there has been no new version of that chart since then. So the corrections listed on the website (www.nmwebsearch.com) are only those for the current edition of the chart.
Within the limits outlined by other posters - yes. Here's the link for the searchable NMs. <A target="_blank" HREF=http://www.nmwebsearch.com/>http://www.nmwebsearch.com/</A>
I still find it a problem however when trying to update Small Craft Folios - you have to find the corrections for each of the charts from which they are derived - I find it a bit of a slow process.