In my opinion, absolutely not. I hate to think that the only reason he killed Stracci and Cuneo was for dramatic movie purposes. I disagree.

Obviously Barzini and Tatt had to go.

Reading the novel and watching GF1, there are consistent hints about Stracci and Cuneo involvement. "The other families will support anything that prevents a full scale war." Couldn't they have backed the Corleones on principle in order to avoid a war? At the five families meeting, the Corleones are muscled into the drug business by the Five families, Stracci and Cuneo included.

Picture a fistfight. Vito shows up to fight Barzini and Tatt. Cuneo and Stracci show up in the same car as Barz and Tatt and stand beside them before the fight starts, clearly showing their support. Even though they don't throw the punches, they clearly show their support for the opposition. Why spare them?

From an ethical standpoint, Moe could have been the only character spared because he did not present a danger to life and limb.