In most cases, no—you do not need to put rocks at the bottom of an urban planter box before adding soil. The old “gravel for drainage” idea is one of the most common container-gardening myths. Rocks do not make water drain better through the soil. Instead, they can make drainage worse by creating a coarse layer under a finer potting mix. Water can pause above that boundary—a phenomenon often called a perched water table—leaving the lower part of the soil more saturated than it would be in a uniform mix. Rocks also take up root space and add unnecessary weight, which matters on balconies, rooftops, and other urban sites.
What your planter really needs is a clear way for excess water to leave. Check that the box has drainage holes at the lowest point. If the holes are small, drill or enlarge them carefully, and cover them with mesh, hardware cloth, or landscape fabric so soil does not wash out. Use a light, free-draining potting mix rather than garden soil; ingredients such as perlite, pine bark, and compost help air and water move through the root zone. Leave a small gap between the soil surface and the rim so water does not overflow, and place the planter where it can drain freely. If it sits on a balcony or roof, use a saucer or drainage tray that you empty, and protect the surface below from staining.
There are a few exceptions. If you are building a sub-irrigated planter or a false-bottom system, follow that design’s specific instructions; that is an engineered water reservoir, not a random layer of rocks. If the planter has no drainage holes and cannot be modified, do not fill it with rocks and hope for the best. Instead, grow plants in a nursery pot with drainage holes inside the decorative box, or choose a planter with proper drainage. For very large raised beds open to the ground, drainage is usually handled by the soil and site, not by gravel. In short, skip the rocks, invest in drainage holes and good potting mix, and your urban planter will be healthier and easier to manage.