When you hook a speaker to an amplifier, you create an electrical circuit. The speaker, now part of the circuit, places a resistance on the circuit. In other words, it takes work/energy to move the cone, this is the resistance shown to the circuit. It is measured in ohms. The greater the number, the more resistance the speaker/coil shows its circuit.
Now lets look at the amplifier. Its output power could be looked at as a pushing force. In an example, a person is pushing a rock up a hill with all his strength. The person represents the amplifier, the rock is the speaker, the movement of the rock up the hill is the work produce (sound created from speaker). Lets adjust a variable and see what happens...
New example, same person, same hill, smaller rock. Remember, rock = speaker. Smaller rock poses less resistance to pushing it up the hill (less ohms). Same guy pushes with same force, smaller rock... net result is faster motion up the hill.
Point? Look at speaker impedance and amplifier force the same way. The amp is pushing with a specific force, its only two variables that change output are input signal strength (not relevant to this discussion) and resistance seen on the output circuit. Still with me? //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif So as you hook up different speakers with different impedances to the same amp, output will scale according to the impedance of the speaker. Ohms = resistance = impedance (generally speaking).
So, your question, does it matter they are different. Well, each set of speakers is hooked to its own amplifier, they have their own relationship going on. The amp for the front speakers doesn't know whats going on between the sub and its amp, nor does it care. It only sees input voltage strength, nd output circuit resistance. These two couple to form the output of the amplifier (in watts). At this point, factor in adjustments for speaker efficiency (sensitivity) and enclosure efficiency, and you have the equation for actual output (very basically speaking here).
Hope that helps.