Short answer: usually no. A layer of rocks at the bottom of an urban planter box is not a reliable drainage solution and is often unnecessary. Drainage comes from gravity, open drainage holes, and a well-draining potting mix—not from gravel. In fact, rocks can reduce the amount of soil available to roots and can create a perched water table, where water sits in the fine soil above the coarse rock layer because it does not move easily from fine soil into large air pockets. That keeps roots wet longer and raises root rot risk.
What matters more is that your planter has adequate drainage holes, that those holes stay clear, and that excess water can escape. Cover large holes with mesh or a drainage screen to stop soil from washing out, but avoid a thick gravel layer. Use a quality container mix that holds moisture without staying soggy; garden soil is usually too dense for planters. Raise the box slightly if it sits flat on a balcony or roof so water can flow out, and use a saucer or runoff tray only if you empty it after watering. Water until a little runs from the holes, then let the top layer begin to dry before watering again.
Urban planter boxes need extra care because they may be on balconies, rooftops, or paved courtyards where runoff, weight, and wind matter. Check holes seasonally for clogging by roots, soil, or debris. If the planter is very large, use drainage cells, a false bottom, or a lightweight filler designed for planters rather than loose rocks. For deep built-in beds, a coarse layer can sometimes reduce soil volume and weight, but it is a weight-management choice, not a drainage fix; separate it with geotextile so soil does not wash into it. If your planter has no drainage holes, do not rely on rocks. Drill holes if possible, or use a sub-irrigated or self-watering system designed for containers.
Rocks can be fine for a decorative top dressing, for blocking a very large drainage opening with mesh underneath, or as part of a specified drainage-cell system. But as a general rule, skip the rock layer. Give roots air, give water a clear exit, and use the right soil mix; that is what keeps an urban planter healthy.