In most cases, yes—or at least some form of drip protection. Urban planter boxes are often watered more heavily than garden beds, and any excess water that escapes the bottom can soak into deck boards, sit in joints, and cause staining, mold, rot, or freeze-thaw damage over time. A saucer, drip tray, or sealed liner gives that water a place to go and keeps the deck surface drier.
That said, a saucer is not the only solution, and it is not always mandatory. If your planter has an integrated reservoir, a raised drainage layer, and overflow holes that direct water away from the deck, you may not need a traditional saucer. The key question is where the excess water ends up. Water should never be trapped directly against wood, composite, or concrete.
For most decks, the safest setup is a planter box on pot feet or a sturdy stand, with a waterproof drip tray underneath and a small gap for airflow. Check the tray regularly, empty it after heavy rain or watering, and make sure it cannot overflow onto the deck. Avoid saucers that stay full of standing water, especially in warm weather, because they can become mosquito breeding sites and can keep the planter base constantly wet.
Weight matters too. A large planter, wet soil, and a full saucer can be heavy, so confirm your deck and railing can handle the load. On composite decks, trapped moisture and grit can still cause staining or surface wear, so risers and a removable tray are wise. On wood decks, consider a waterproof barrier plus a tray, and inspect the area underneath for dampness.
If you prefer a no-saucer look, use a self-watering planter with a closed reservoir, a custom waterproof liner, or a decorative outer pot with a hidden drainage tray. Just do not let drainage water run freely onto the deck. In short, saucers are not absolutely required in every design, but for most urban planter boxes on decks, a tray or equivalent moisture barrier is a small investment that protects the deck and makes maintenance easier.