Saccharin is made from petroleum, via a complicated chemical reaction. Saccharin is rated to be stable at 200 degrees Fahrenheit, however if it touches the wire in your atomizer it may exceed this temperature especially if there isn't much e-juice left. The wire can get so hot that, as most of us know, eventually the metal wire itself burns out. Saccharin was never intended to be exposed to the temperatures found in an atomizer coil. I wasn't able to find any sources on what exactly happens when Saccharin is exposed to more than ordinary cooking temperatures. I'm not even sure if this has ever even been researched at all.
With chemicals made in a laboratory, it's difficult to know exactly what happens when it gets exposed to very high temperatures. Too much regular sugar or high fructose corn syrup would likely caramelize and gum up your atomizer after a while but should be otherwise safe. The best option would be to use Stevia packets, this is an extract from plants so it shouldn't turn into anything toxic if it gets hot. The Splendex mentioned in some other posts should be fine as well, it's made from sugar.