MRU is the default policy for Logical Unit Numbers (LUNs) presented from an Active/Passive array.
Fixed is the default policy for LUNs presented from an Active/Active storage array.
see if this KB helps.
The policy also depends on the storage array you are using for your raw device mapping.
You have to check the documentation of your storage vendor to see the best practises...
Are you using MSCS? From vSphere 5.5 the Round Robin path selection policy is supported for the shared RDMs as long as your storage supports RR. Prior to 5.5, RR was not supported for the shared RDM in a MSCS configuration so if your storage defaulted to this you should change it. See VMware KB: Microsoft Clustering on VMware vSphere: Guidelines for supported configurations
All depends on your storage, and your storage vendor should have some implementation recommendation for vSphere.