SSAS and SQL Server on the same server
Why is SQL Server on the same box?

"Perform Volume Maintenance Tasks" should certainly be granted to the service account used to run SQL Server since that will allow SQL Server to not spend time unnecessarily zero-ing files during expansion of existing files and creation of new files.

"Lock Pages in Memory" would be a good consideration if you want to guarantee SQL Server has enough memory to perform the workload assigned.  If you don't care about the SQL Server workload, why is it on the same box as SSAS?