Best practice based on RecoverPoint and Riverbed joint testing is to set compression in RecoverPoint to 'None' in order to allow the compresion algorithms in Riverbed to function as effectively as possible.
At Riverbed we just finished a new technical white paper in collaboration with EMC.
This speaks to the benefits and best practices of the combined solution. I can't seem to attach docs - so I'll provide the link to this on our Website:
(Also available if you look under http://www.riverbed.com/emc under Documents).
Hope this helps.