If you have to prune it in order to keep it to a certain size, or if it's getting very leggy, then do so immediately after flowering. You can cut any or all stems back to a healthy, visible growth bud to keep to whatever size and shape that you need.

Having said that Tree Paeonies don't actually need to be pruned at all and will not grow to more than 5 or 6 feet, so unless yours is outgrowing it's spot then I'd leave well alone.