Here's the bottom line. Feel free to pour boiling water down any drain that is unclogged or not made of plastic (or PVC, a type of plastic used in plumbing). If it's clogged, the boiling water will be trapped and cause the plastic to soften. This can lead to a burst pipe or leaks from the pipe or the seals and fittings. If a metal drain (such as steel, iron or copper), there's no harm using boiling water, even if the drain is clogged.