NFS datastores do not suffer many of the caveats that block storage protocols do, mainly because the storage array has visibility into the specific VM a write or read is being issued to.
Assuming the storage array is using a single volume to host the NFS exports, I typically had NFS datastores configured for specific types of workloads for replication, backup, and deduplication, but not performance. Since the same back-end spindles were used, any number of NFS exports on the same volume would all share performance power, anyway.
I still like splitting out the OS partition from other application / backup partitions within the guest, mainly by way of unique VMDKs. You might split out the NFS datastores this way, too, but again - this would mainly be for backup and replication. Such as creating a policy to backup the OS export on a different schedule than an application export.
Also, refer to your storage vendor's documentation. It typically contains their recommendations on storage volume / export layout, along with maximum volume sizes.