Matching A/V Receiver/Speaker Impedance

How important is it to match the impedance of the speakers to the receiver? If they don't match, will it cause harm to either? Will it lessen performance? Can you safely use speakers of differing impedance in the same system?
The standard for A/V receivers has been 8 ohms.Using speakers lower than that can be hard on the receiver.If a receiver is over driven,it can clip causing damage to the speakers,and or receiver.A 4 ohm capable receiver is rare.♫