any 220v appliance is YES safe to use at 240v. if it is heater or range, the wires may be a little hotter; if motors, it may run a little slower and less efficient.
From safety point , it is NOT very good. in Canada, the Zero-hot are different size outlet, so the appliance will have a safe ground point, but there is not ground for 240V voltage (so 240V mostly use for range).