I agree with Nigel Goodwin.
Unless you're dealing with RF (and VHF, and UHF, and so on), in which case impedance matching is of EXTREME importance.
In RF designs, if the impedance of the load doesn't match with the characteristic impedance of the "line" (the line can be thought of the circuit itself, its wires and patches and stuff), there'll be reflections across the circuit that can ruin everything.
In transistor circuits and so on, it's more important that the signal doesn't get attenuated along its path. So, for voltage signals, the input impedance of your "amplifier" (understand amplifier as "signal conditioning system") should be much higher than your source impedance. Conversely, the output impedance of the amplifier should be much lower than the load impedance, so that most of the signal stays on the load, and not on the amplifier.
For current signals, everything is exactly the opposite.
edit: Unless, of course, you're interested in power transfer. If you want to transfer
power from one system to another, their impedances must be matched (didn't said that before because I think you already know this)