My thoughts - on older heads anything that reduces surface area where carbon can anchor to it, definitely (the quality is usually not great - porous castings etc). But there's a bit more to it. You got lots of sharp edges in there around the valve reliefs and the spark plugs and those edges can potentially do the same thing. Just radiusing them off will also aid in reducing turbulence of gas going in and out and can help with fuel ignition propagation (the fuel charge can pass through the spark plug relief instead of sort of bouncing around it). By looking at the chambers you can imagine how the gases swirl into and out of the valves so you want to speed up that process.
Better cylinder charge fill and improved burnt gas scavenging = more efficiency.