A little stand-alone refrigerator thermometer is so inexpensive, there's really no excuse for not having one. Especially when it can do all of this for you:
You can figure out the cold and warms spots in your fridge. Yeah, it should keep everything at a chilly 35°F, but refrigerators usually have some variation in different spots. With a thermometer or two placed inside, you can figure out the best spot for everything.
You'll have a heads up if your refrigerator or freezer is on the fritz. If your aging appliance is getting weaker—or quits on you—you'll know it a little bit sooner if you keep a gauge inside.
Stay food safe in an emergency. Power went out because of a storm? You'll know if it's time to toss your fridge or freezer full of groceries by monitoring that the temperature inside is the right temperature for food safety.