Hi tarttartan, definitely don't prune your cherry tree now. This job needs to be done in summer, to prevent the fungal disease silver leaf. You could tie ribbons around the branches you want to cut now, so you know what you're cutting in summer.. but otherwise, leave well alone.
Regarding your pieris, I would also wait a while. In winter, there's the potential for pruned plants to get die-back, so you'd have to prune all of this out anyway at a later date. Wait until temperatures increase and prune your pieris in spring, instead.
Hope this helps
Kate
gardenersworld.com team