Don't automatically assume the PK makes the best, or even a good, clustered index, in particular if the PK is an identity column.
You need to review the existing index usage, missing index info (but don't blindly accept what it says!), and understand the most common query type(s) on that table before deciding on the correct clustered index column(s).
SQL DBA,SQL Server MVP(07, 08, 09) Prosecutor James Blackburn, in closing argument in the Fatal Vision murders trial:
If in the future, you should cry a tear, cry one for them [the murder victims]. If in the future, you should say a prayer, say one for them. And if in the future, you should light a candle, light one for them.