Otherwise you can let the system and disk handle any over provisioning or redundancy. Nowadays the only reason to keep free space is to ensure that applications that need to write to disk can. You're right, but that argument is also completely irrelevant when considering SSDs. That's just wrong, and is especially aggravating since storage is now measured in SI-prefix gigabytes so you're actually only getting 112 imperial GB's worth of storage, minus however much space you're willing to sacrifice for performance.
To the OP: I was sure that Apple sold these as having 120GB capacity, but you're right, they claim 128GB (with a bullshit 'actual formatted capacity less' disclaimer). On the plus side, this redundancy is shared by the hardware and OS. It's not a hard requirement: everything will work even when filled to capacity, it'll just be slower. Keeping 10-15% space free on your disk helps the filesystem keep fragmentation down, and the more unused space you have the easier it is for the flash controller to maintain good performance. You'll still want some space, as various temp files and other things need to be created during normal system operation. With the memory compression in Mavericks, much less paging space is needed, so you may be able to cut that a bit closer. A 10% reservation for the system was mainly desirable to accommodate memory paging.
