It depends on the file size of the files stored on the drive. Small files go faster with small block size and large files with large block size. So for instance, a web server shoud have small blocks (many small html and image files). If it's the drive you store movies on (and only that size files) you might consider a larger block size. But 64k is a good allround size.
Or maybe I've confused the whole thing since this is about RAID... :/