Is it safe to leave my mining PC running 24/7?

Generally yes, with sensible precautions. A modern desktop is designed to run continuously - servers do it for years. The risks at home are dust buildup, heat, and a poor-quality power supply. Clean the dust out every few months with compressed air, monitor temperatures with a tool like HWMonitor or lm-sensors, and make sure your PSU is from a reputable brand and not overloaded. Use a surge protector or a UPS if you have unstable mains power. Do not leave a laptop mining unattended for days at a time - they are not built for it. And keep the room ventilated; even a single PC adds noticeable heat to a small room in summer.