No. It's not safe. You are not a thermostat. You can't be there constantly checking and adjusting your temperature the way a thermostat would.
Now, if your probe is affixed on the inside of the bottom tray under the substrate, then the thermostat should turn off your heating element before it gets too hot in the enclosure. In your original post, you said that temperatures sometimes get "way higher" than 89 degrees. Exactly how high do the temperatures get?