Interesting question. Here the berries and flowers are most commonly used, but I know the leaves and bark are used in medicin too. If I dare say anything; I know an acknowledged herbalists gives a recipe for a tincture of leaves and flowers, and both are picked during the bloom, chopped up and used fresh in alcohol. For many plants the best time is to pick new growth, before mid summer, but since this a small tree it might be different. I am sure you know the general. I personally would pick new growth unless I could find some detailed studies on it. I don’t know if the relevant subustances in the flowers are reabsorbed, but I would assume something is not ideal if the tree blooms and there aren’t any berries later on.