Without spoke head washers the heads sit all pissed in the larger holes which introduces movement/fatigue.
You can probably build a wheel without them and get away with it, for a while at least, but they’re neither expensive, hard to get or difficult to use so why take the risk?
Butted spokes are probably all you need; the extra elasticity should prevent the fatigue failures.
99% of spoke breakage is caused by the loosest spokes losing all tension as they pass by the road.