No, it's never a good idea to heat the water directly into French Press Jar. There are two problems with it. The first one is that the material of most of the French Press Coffee Makers is not suitable for exposing it to direct heat. If it is made of plastic, it will be deformed if it is made of glass, dark black stains will develop at the bottom, or it might get cracked. If you have a steel French Press Coffee Maker, you can put it on the stove. But there is another problem with it. The temperature of boiling water is 212 degrees which is not suitable for brewing coffee.