Occasionally using hot or near-boiling water to help clear light grease is usually fine, especially if you have metal pipes. But doing it very frequently or with extremely high temperatures can stress some plumbing, particularly plastic PVC pipes and old joints.
Boiling water alone won’t fix serious clogging caused by hardened grease, food chunks or foreign objects. In fact, if the grease has already solidified further down the pipe, the hot water may only move the problem deeper instead of removing it.
A better habit is to avoid pouring large amounts of oil or fat down the sink at all. Wipe greasy pans with a tissue or scrap paper before washing. Use a sink strainer to catch food bits. For maintenance, occasional hot (not violently boiling) water, plus dish soap, can help.
If your drains clog often, it’s smarter to use a drain snake or call a plumber, instead of relying on extreme water temperature or harsh chemicals. So, small amounts of hot water now and then are okay; using boiling water as a daily “treatment” isn’t ideal for the long-term health of your pipes.