What's the amp?
Nominal impedance is what it says - a nominal figure for the purposes of comparison. All speakers have impedance that varies with frequency, to a greater or lesser degree depending on the design. The ideal is a ruler-flat impedance across the frequency range, just as a ruler-flat frequency response with no spikes or dips is also the ideal, but in reality this is all but impossible to achieve, and it's not unknown for a speaker with an 8ohm nominal impedance to dip as low as 2-3ohms at certain frequencies.
So if you have a speaker with a nominal impedance of 4ohms, say, I'd select the 4-6ohm setting available on some AV receivers to protect the amp, or the 4ohm taps on a valve amp offering 4ohm and 8ohm connections. With speakers of a nominal 6ohms I wouldn't worry too much.
And BTW your speakers don't offer or deliver 120W - the figure just means they can handle an amp delivering 120W, but again check whether this figure in the specification is a peak value or a continuous one.
In other words, a speaker able to handle 120W continuous would be fine with a 120W amplifier running flat-out, whereas one able to handle 120W peak would be better used with a less powerful amp.
However, it's much easier to damage speakers by running them on the end of an amp working so hard that it's clipping, rather than one with a power output greater than the speakers' nominal power handling that's only operating at a modest part of its output.