Encoding NULLs as sentinels/extents

Hi gurus,

    Through dim distant memory, I seem to recall being warned at uni never to encode a null in a field as a value.... strangely enough, on my current project we have a huge number of date range fields that are, (you guessed it!), currently encoding NULLs as "infinity" (ie. from here till eternity). Hence to find those records that were / are / will be current for a specific date we have to do something like.....

SELECT     *
FROM         HISTORY_TABLE  H
WHERE     <PARAM_DATE> >= H.DATERANGE_START
AND            (( <PARAM_DATE> <= H.DATERANGE_END OR H.DATERANGE_END IS