Depends on what you want from them. If they produce catkins, then the time to prune is after the catkins have finished. THen they will spend the Summer producing new shoots on which the catkins appear next Spring. Pruning for shape and size can be done at time time convenient to you, though I would wait until frost danger is over.
Robinias are not usually pruned at all, but again if you need to reduce the size, any time after last frost.