Peter_s - you're right about very short runs. Connection is usually considered a transmission line when output transition time is less than 8x propagation time. For typical 25ns transitions found in less expensive transports, assuming 0.7 speed of light, it will be about 2 feet. It might be a serious problem with more expensive transports that can swing in 5ns - our distance would be less than 5". Once we get transmission line effect (reflections on characteristic impedance boundaries) it will be wise to make cable longer to avoid reflection to interfere with the same edge (fastest slew rate is at the beginning of transition). Reflection caused by impedance mismatch (that always happen) coming back fast might interfere with the same transition (creating jitter) but won't be strong enough to cross the threshold in the area after transition - that's why longer (1.5m) cable might be beneficial. I keep it very short - about 6".
It is system (and environment) dependant. Expensive digital cables will make big difference in some systems but won't make any in others. Even Toslink might be good for some systems because it is not creating ground loops.