There is no universal agreement on this issue. However, most people seem to fall on the -long ICs and short speaker cables- side of the fence. If we are talking the difference between 12 ft. of speaker cable and 18 ft. of cable, then I don't think it makes any difference...just keep both cables equal in length.

If we are talking 20 ft. or longer runs of speaker cable, then I would go with up to about 3 meters in IC length instead and opt for the shortest speaker cables that would get the job done and still allow a little flexibility in placement.

In the event that your amp has a higher than normal output impedence for a solid state design, I would go short speaker cables no matter what. That wouldn't be common for solid state amps, but very common for tube amps.