if you are worried about heat from the bulb, just reach over and feel them. If they feel warm to your hands then they probably feel warm to the betta.
Temp changes that will severely damage the fish's health would be things like your room being 75F and then you turn the heat off and open a window, suddenly your rooms like 32F. Then you have a lot to worry about since the water temp will be dropping rapidly. However, in most cases, if your house is anything like mine, your temps during the day are around 70-75F and at night it drops to 60-65F. This is ok. The temp in your water will drop but not super quickly as the temp in your room doesnt suddenly change. With the small Hydor heater i see temps around 80 during the day and around 70 during the night, which is fine for the betta.
My betta is slightly more sluggish in the cooler temps in the morning but maintains his color and actively eats, swims and responds when i approach.