Indexing would make no difference. The only reason that the exhaust valves ALWAYS have less residue and build up on them is because they operate at a much higher temp than the intakes. The intake valve is closed while the combustion gasses and heat are swirling past the exhaust. On the intake stroke, the atomized gasoline and cold air keep the intakes at a much lower temp than the exhaust, which is conducive to the build up of carbon.
It can get really bad depending on the engine design, interference engines in some German cars can and do seize because of the buildup on the valves and pistons, when you take the head off or look in the cylinder with a boroscope, the exhaust valves are relatively clean while the cooler parts like the piston and intakes are coated deep.
It has absolutely nothing to do with how the spark plug is oriented. Some MD 20/20 down the intake will probably do more good than indexing the plugs, but then you'd have to replace the exhaust where it'll eat through the chrome and H-box.
