In most cases, yes. If your urban planter box has drainage holes, a saucer or drip tray underneath is a smart way to protect a deck. It catches the water, soil particles, and fertilizer runoff that would otherwise drip onto the deck surface. Over time, that moisture can cause wood to warp, rot, or grow mold; it can leave rust or tannin stains on composite decking; and it can create slippery, dirty patches on concrete or stone. A saucer also makes cleanup easier, because you can remove and rinse it instead of scrubbing the deck.
That said, a saucer is not a magic solution. If water sits in the saucer for days, it can keep the bottom of the planter too wet, encourage root rot, and breed mosquitoes. It can also trap moisture against the deck itself, especially if the saucer sits flat on the boards. So the goal is not simply to add a saucer, but to manage drainage well.
For a deck-friendly setup, choose a saucer or drip tray that is slightly wider than the planter and deep enough to hold runoff. Empty it after heavy rain or watering. Better still, raise the planter on pot feet or a small stand so air can move underneath, and place the saucer so it does not sit directly against the deck for long periods. If the planter is heavy, use a liner inside the box and a sturdy tray underneath rather than trying to move the whole box every week. Check the drainage holes regularly so they do not clog with roots or compacted soil.
There are a few exceptions. Some self-watering or sub-irrigated planter boxes have an integrated reservoir and sealed base, so they may not need a separate saucer. However, you should still check for overflow during heavy watering or rain. If the planter is on a balcony with a drain and you do not care about staining, you might skip the saucer, but the water will still go somewhere. In shared or rented spaces, a tray is usually the safer choice.
For wooden and composite decks, a saucer is especially recommended. For concrete or stone patios, it is less critical for rot but still useful for preventing stains and mess. In winter, empty the saucer before a freeze so trapped water does not crack it or keep the planter waterlogged. If mosquitoes are a concern, empty standing water often, or use a tray with a grate and add a mosquito dunk if needed.
The short answer: yes, most urban planter boxes should have a saucer, drip tray, or liner underneath to protect your deck, but you must empty it and keep airflow around the planter. A saucer protects the deck best when it is used as part of a complete drainage routine, not as a permanent puddle.