I heat mine on the stove in a SS pot. I sometimes, umm, forget that I’m melting my fats and, well, well, let’s just say that this is how I discovered that your fats can get REALLY hot and are fine in soap.*
I use the stovetop mostly because I master batch my fats and the volume won’t all fit in the microwave. The whole batch goes into a large plastic tub that rests on a heated mat, covered with a towel. The fats are a consistent 95-98 degrees until I need to make soap.
*this is also why I frequently burn my morning oatmeal.