As a rule cherries and plums don't like being pruned. They leek sap, can be prone to infection via the wounds, and often grow back awkwardly. However, a careful trim of leggy stems back to a smaller side branch/twig in spring will keep a smallish tree in check. Tibetan Cherry is a small tree anyway, so you may find any pruning unnecessary. I have taken quite sizeable pieces out of mine (12ft tall), throughout the year, without any ill effect on the tree.