According to some literature when I researched this topic, the total resistance of the wire should be less than 5% of the nominal impedance of the speakers. Of course, the speaker impedance can dip well below the "nominal" value at certain frequencies so the safer bet is more like 2-3% depending on the impedance curve. Furthermore, the amplifiers with multiple speaker impedance taps, e.g., 8, 4, 2 ohms, etc., tend to have a different damping factor (output impedance) for each tap. So sometimes it might be worth to experiment with the different taps, especially when running long cables, to decide which sounds better. For example, my MC2200 manual lists the following damping factor values:
DF 16 for 1 ohm
DF 50 for 2 ohm
DF 30 for 4 ohm
DF 16 for 8 ohm
In my set up where I need to run very long speaker cables (true bi-wire 12awg), I get an overall better SQ when I use the 4 ohm tab for my 6 ohm speakers but at the expense of a narrower/deeper soundstage when compared to the 8 ohm tap. Interestingly, the 2 ohm tap has a slightly fuller sound but it compresses the soundstage even more considerably. My 2 cents FWIW.
DF 16 for 1 ohm
DF 50 for 2 ohm
DF 30 for 4 ohm
DF 16 for 8 ohm
In my set up where I need to run very long speaker cables (true bi-wire 12awg), I get an overall better SQ when I use the 4 ohm tab for my 6 ohm speakers but at the expense of a narrower/deeper soundstage when compared to the 8 ohm tap. Interestingly, the 2 ohm tap has a slightly fuller sound but it compresses the soundstage even more considerably. My 2 cents FWIW.