ideally you should have a relay that switches the speaker between the amplifiers output, and ground. In a fault condition, that makes it much more likely that the output will travel to ground, even if it arcs, rather than continue through the speaker.
If you short the speaker wires out, the relay will give protection whether its placed at the amplifer output or in the speker return line.
Hhowever, if you have the relay in the speaker return line and you accidently short the amplifier output to the chassis, you are bypassing the relay - no protection.
I am using solid state relays, and in one of the designs I have put the relay in the speaker return line because its easier to do - but, the only protection if I short the amplifier out put to the chassis is the fuses (Which Dave Zan kindly pointed out to me :0).
BTW, be carefull about assuming an electromechanical relay can save your speakers if you get a catastrophic failure on your amp . . . . do a search on the forum.