quick question regarding proper use of indexes.

there are options for asc/desc sorting for each column when creating the index
- for optimal performance should i create a new index for each sort possibility?

i'll use the coldfusion forums as an example.
if i wanted to sort threads by newest to oldest, i would create an index that
uses post_date desc
now if i also wanted the performance gain of an index when sorting by oldest
to newest, would i also create an index that sorts by post_date asc...

thanks