It's not necessarily harmful, and it depends on your situation. Until very recently I cooked on a glass cooktop, and if you go straight to high with a cold pan

It's not necessarily harmful, and it depends on your situation. Until very recently I cooked on a glass cooktop, and if you go straight to high with a cold pan you have a high risk of warping it (which you'll know if it spins on a flat surface) which makes it heat unevenly. If you use a gas cooktop, you kind of don't have to care as much. A warped cast iron pan still can cook just fine on gas, but it won't work as well on a ceramic cooktop. Low temps guarantee you won't warp or crack the pan. But if you're not super worried about it, go ham.