It's kind of funny, and potentially a bit unsettling, how spread spectrum on motherboards came about. To simplify a bit, modern systems emit a lot of energy, so to get around FCC limitations, spread spectrum (extreme simplification here) spreads the energy across a wider range of frequencies. The FCC test at (IIRC) 125hz increments, so by spreading the energy out across a wider band, each 125hz segment remains under the emission limits. The potentially unsettling part is that systems are still releasing more energy, it's just spread around rather than concentrated at a certain frequency.
As it applies to the user, disabling spread spectrum (a function the FCC tolerates as long as it defaults to 'on') may increase stability in more extreme overclocking. Practically however, it's not likely you'll notice any difference at all with it on or off, so best to just leave it on.