So, I tried it both ways. Setting opacity to .501 gives it a richer look, I think. But turning off shadows lessens the effect of leaves floating. I spent several hours sitting on the back porch with a glass of wine, studying piles of leaves (wife says I should stop napping, put down the wine and rake the damn leaves - she clearly doesn't appreciate art) and think that leaves occupy a definite 3D volume with random small shadows that erase the flattish look. So I'll set the leaf opacity to .501 and re-render.
