Clustered Index Selection - PK or FK?
Community wiki answer:

I think a composite clustered index key with CustomerID as the first column will be best since that's in the WHERE clause of nearly all queries.

There may be more splits compared to an incremental key (or more likely suboptimal page density for a time if you manage and maintain fill factor to avoid 'bad' splits). However, the overall performance improvement for customer queries is substantial, because the key lookup is avoided.

OrderID or OrderDate may be best for the second column depending on your most critical queries.

For example, if customers see a chronological list of recent orders after logging in to a web site, OrderDate should be next, to optimize ORDER BY OrderDate DESC.

If you choose OrderID as the clustered index, with a non-clustered index on CustomerID, you'll still get splits and fragmentation, just in the non-clustered index.