Not sure about a mandate, as a clustered index relates to how the data is physically stored on disk, hence you can only have one. Given that, it's best to have your clustered index on column(s) that will give a benefit in that regard.

My own preference is on column(s) that are required in common range lookups. So for unique IDENTITY() columns: No. But for datetime columns, or a "category" column, then you get the benefit of the physical order when reading pages BETWEEN <here> and <there>.

S.