Tempered glass is not only safe, it is also heat resistant since it has been through so much heat at the manufacturing stage. It is no surprise that shower glass panels and insulated glass windows are all constituted of such glass since these objects are constantly subject to high temperature spikes.
Tempered glass can withstand heat up to 470°F (or 243°C for you Canadians out there). Hence, the perfect applications for tempered glass are shower doors and panels, windows, and exterior glass railings – all applications where high temperature or frequent temperature variations are involved.
You can use tempered glass for oven or fireplace door replacement, but ceramic glass would be the preferred glass type in such a situation. Avoid using tempered glass for woodstoves, since wood combustion happens anywhere between 572°F and 1,112°F (300°C and 600°C).
If your tempered glass is exposed to temperatures closer to, or over 500°F (260°C), your piece would weaken significantly over time and become subject to breakage.