How does order of index affect performance in SQL Server?
In this article someone looked at the performance of an index and the ordering of the data in the index.

Summary
As we have shown creating an index in ascending or descending order does not make a big difference when there is only one column, but when there is a need to sort data in two different directions one column in ascending order and the other column in descending order the way the index is created does make a big difference.