From a purely querying standpoint, it makes no difference whether your key is descending or ascending if you want to pull N most recent or N oldest records:
The internal algorithms of SQL Server can navigate equally efficiently in both directions on a single-column index, regardless of the sequence in which the keys are stored. For example, specifying DESC on a single-column index does not make queries with an ORDER BY IndexKeyCol DESC clause run faster than if ASC was specified for the index.
http://msdn.microsoft.com/en-us/library/aa933132(SQL.80).aspx
However under just about any normal circumstance, you want your primary key to be ascending and ordinally sequential to prevent fragmentation. SQL Server is optimized for physically appending new records to the end of the database file. If it needs to insert each new record at the top and push everything down, it would probably result in nearly 100% fragmentation.