S_A: If I interpret your question correctly, my answer would be, that this would depend on the physical arrangement of the involved components as well as whether you'd use self-made y-cables, ready-made y-cables (and which type (short/long split)) or cables plus splitters and also on the particular components. Assuming a typical arrangement (pre-amp farther away, the two power-amps (or each pair of the four mono-blocks) closer together), you could certainly save some cable length by splitting at/closer to the first power amp/mono block instead of at/closer to the pre-amp - though at the cost of having at least one more contact transition on the way to the second power amp, unless you'd use/make yourself a short-split y-cable.
Or in other words: I could only tell you, which approach I'd prefer in your situation, if I knew more details about your intended set-up. However, in practice the differences wouldn't really be that significant anyway - and then, what would seem most ideal from a technical point the view would still not be guaranteed to also be most to your taste.
Greetings from Munich!
Manfred / lini