Basically, if you are going to have a lot of small files on your filesystem, choose a small number of bytes per inode. Otherwise the default will work fine.
Basically, each file requires 1 inode (this holds the metadata such as permissions and the list of physical disk blocks the file is stored on) + whatever data blocks the file uses. So if you have a bunch of small files, you risk exhausting the inodes before the filesystem is actually full space-wise, which is bad, since no free inodes means no new files can be created. The larger your inodes, the fewer there will be, but the more data about the file (i.e. bigger block lists) can be held in each inode.