You cannot wire two speakers together so that the impedance is the same. You will either end up with half the impedance (parallel) or twice the impedance (series).
For series wiring, the speakers are inline with eachother (+ to + speaker A, A- to speaker B+, B- back to amp), so the impedance is added together - two 8 Ohm speakers become a 16 Ohm load which is easier to drive but you also lose output.
With parallel the wires are either all connected directly to one channel or from one speaker to the other, keeping + to + and - to - consistent. In this case, the the impedance is halved (assuming 2 speakers) - two 8 Ohm speakers become a 4 Ohm load, which is literally twice as hard for the amp to drive. If your amp isn't 4 Ohm stable, then yes it is very important because you could damage the speakers or possibly even the receiver.
To do this for a center, the safest way would be series, and then you will have to raise the output of that channel in the speaker setup menu.
