generally I stick with the 20% rule for NFS volumes 1TB or smaller and if they are data stores for the system partition of multiple guest VM's. For a NFS volume that holds only a single VMDK of say a 500GB data volume for a file server then I only leave 10%. I run a 3rd party backup util and some free space is necessary if you snap a vm for either a backup or manually in virtual center. For larger than 1TB NFS volumes I generally use 10% free space and this has been fine so far. If you have VMDK's all thin provisioned then you really need to know what your disk commitment is and the probability of a large infusion of data to any VMDK that might fill a volume. (Be careful of over commitment and the provisioned size of your disks). For that I use auto grow to a max of 10% original size in 50% increments. I manage roughly 95TB across 2 data centers running 3140 and I have been ok so far.