By default some space is reserved for root only, i think about 5% or so. See man tune2fs for the -m option. I personally would leave it as is.
I was looking at one of our RHEL 5.7 systems here and I did a df -h to see the file system sizes and I noticed something that I found odd. One of the mount points shows total size of 549 GB, total used is 2 GB, 1% utilization, but only 519 GB available. Where did the rest go? That's a pretty big discrepancy. This particular file system is LVM using ext4, but the rest of the partitions are the same and the numbers aren't way off like on this partition.
Kevin