From my understanding, spread spectrum controls the EMI emissions that your motherboard CPU clock gen produces. Since the clock gen of your CPU is oscillates at whatever speed your CPU is running at and at that frequency your motherboard can start giving out EM radiation. That's why cases, with the exception of the clear cases, are EM shielded. What spread spectrum does is reduce the amount of EM rad your mb produces, but can have affects on the signal integrity in the CPU and thus may limit your OC.
BTW, don't worry about the EM radiation, its not harmful.