Unfortunately, any stem which is leaning now will grow straight up, producing an unnatural elbow where the tip was when planted. It's more of an aesthetic thing: a few like it, but most consider it bizarre! If they are all leaning the same direction, have him come back and replant them. Leaning by itself won't hurt them, but it will take three years of intensive pruning to replace the leaning stems with new straight ones. If it's just a few stems leaning out from the main clump, just cut them off after it has established itself for a few months. If the stems are leaning widely out in all directions, the rootball may have been severely disturbed when they were planted, and you may want the landscaper to replace them, especially since their chances of dying are pretty high!