It is usually recommended if you had an SSD and you loaded your OS on it.
It would help, not as much as whats mentioned above, but it would help if you had a small SSD for ssd caching. But if you can only choose one, go with the OS on the SSD.
Though it is fine, I personally recommend a 240GB SSD, if you are going to load the OS and other programs on the same drive, or a seperate 120GB+ SSD for you to load other programs other than the OS. But if you want the upmost performance, you want to try to keep the OS all by it's lonesome on the SSD.