Never let your plants drown or drought. Slightly moist soil is your best bet. If the ground is saturated, the plants will rot at the roots. Not enough water will cause dessication. In the heat of summer, especially, keep an eye on the plants. The curling and drooping of branches and leaves is an urgent indicator: Water!
Watering in the morning or in the evening ensures that the most amount of moisture is reaching the plant and not just evaporating.
Your plants are newly installed and have nothing to keep them from shifting around: no roots to hold them firmly upright. Though installation is always done properly, outside factors such as wind, kids, pets and wildlife can cause damage or displacement of the plant. These culprits can rub up against, step on or urinate upon the traumatized plants and be a major cause of death or disfigurement.
Try to ensure that the plantings receive the least amount of disturbance within the first few weeks following the installation. As they get established they will still need to be checked for damage from insects, critters and acts of God.
As I tell those clients who want a maintenance free landscape, go plastic so I tell you: plan on it. If you want to preserve the integrity of the plants and your investment you need to anticipate an annual/bi-annual mulching and cleanup at a minimum.
Usually through the growing season you will find it necessary to perform (either yourself or your chosen professional) regular weekly or bi-weekly weeding. The weeds deplete the soil, sap water from the plant and sometimes actually crowd out the desirable plant. It is important to stay on top of the invaders before they become a problem.