The short answer is that you shouldn't use something like NFS for session management because it doesn't handle locking correctly on the distributed session files that could be accessed by any of the client servers. Plus, a lot of solutions don't correctly handle asynchronous reads.
Plus, if your networked file system "goes away", you will be in a real mess, because you get into a situation where you need to start detecting the bad mount in your PHP application.
That said, you could roll out NFS for session management if you expect low throughput, but to be honest, there are better solutions out there.
If I had to rank... I'd say any of the in memory stores are best. Memcache/Redis.
I like using Redis with 5 minute writes to disk.