Hi!
From docs about SRM 6:
from Install guide
If you use vSphere Replication, do not select a placeholder datastore that you already use
as the target datastore for replications.
from admin guide:
The replicavirtual machine that vSphere Replication creates and the placeholder virtual machine that
Site Recovery Manager creates can both reside on the same datastore on the recovery site because they are
created in different datastore folders. When the replica and placeholder virtual machines are in the same
datastore, Site Recovery Manager creates the placeholder virtual machine name by using the replica virtual
machine name with the suffix (1). Site Recovery Manager applies the inventory mappings to the
placeholder virtual machine on the recovery site.
so, I guess the best practice is mentioned in the Install guide 