Most rabbits can handle up to 80F, which would also kill your heating bill anyway
My three start to show signs of heat exhaustion around 75F so I always keep temps below that. For winter, we keep our heat around 60F to 65F. In summer, we keep the AC set to 70F
Note, that rabbits cannot handle more than a 10F temp change very well. You risk them getting sick if you raise or lower the heat too fast. When it comes time to kicking the heat on, try not to increase it more than 10F every 24 hours at the most until you reach the temp you want in your living space.
Edit: signs of heat problems: Drooling, excessive wetness around the mouth/nose, lethargy, heavy breathing, rapid breathing, (when its way too hot) seizures. If you notice any of these signs or fear your bunny might get too hot with your heat on, you can always add ice packs to his space or a fan on half of his space so he has an option to cool off 