Late summer is recomended as the best time for pruning ornamental cherries (Never! winter, As Monty Don said last night)
It is possible to prune weeping forms but try to thin-out the branches individually and cut with your secautres just above a bud (actually below if its hanging down)
I've seen what I call "puding basin jobs"in quite a few front gardens on my travels, where someone has taken the shears to then and they've ended up with a hideous beach parasole effect, but amazingly they survive! Good Luck Tbird+