Do you need coolant flush if engine isn't overheating?

The reason for a coolant flush and change is because the anti corrosion part of the antifreeze loses effect over time. So the coolant starts to eat away at the internals of the cooling system The radiator is the usual item to rot, resulting in leaks. In addition sludge forms which often starts to block the heater matrix and eventually the radiator. I my advice is don't leave it  too long to either DIY or have a shop do it.

don't touch you will create problems

For $20 you can buy a gallon of antifreeze. Just do a drain and fill and you'll be fine. Watch a youtube video showing how to change your antifreeze on your car and learn how to purge the air from the system.

AND ALSO>>>>flush and change the BRAKE FLUID as it becomes LIQUID SANDPAPER as it deteriorates! AND the STEERING FLUID and an AC tune up as well! Go to the DEALERSHIP and HOPE they are using the PROPER fluids to change ALL OF THEM! ASK if they use VALVOLINE! if they do RUN AWAY! CHANGE the TRANS FLUID, filer and GASKET! It is OVERDUE times TWO!  The TOYOTA dealership should have the EXACT RIGHT equipment for ALL these things! AND run a SPECIAL to do the procedure! TOYOTA uses a GERMAN COOLANT that is RED and comes from the PENTOSIN company. TOYOTA uses mostly AISIN WARNER JAPANESE TRANNIES! AND THEY need AISIN WARNER trans fluid from the DEALERSHIP! >>>YOU CAN wait a BIT! YES!  but at LEAST you are AWARE of this thing! IT is TIME to TUEN IT as well! Use DENSO TWIN TIP IRIDIUM LASER spark plugs at $7 each from ADVANCE AUTO! TRUST ME! There is NOTHING any better and it provides more PROTECTION for the EXHAUST system and the CONVERTER and the O2 sensor as WELL! EXPECT to get to 500,000 miles! USE PENNZOIL ULTRA 0W=-30 OIL ONLY made from NATURAL GAS and NOT crude oil! BYE NOW! I have LEARNED these thing slowly while doing LOTS of RESTORATION WORK! on LOTS BMW cars, old BENZ cars, and OLD european cars!  BYE NOW!IT is MOST important to FLUSH and CHNGE the BNRAKE FLUID when it is VERY DARK BROWN< to protect teh ABS PUMP the CALIPERS from FREEZING UP and the MASTER from deterioration! BYE NOW! 

That kind of coolant flush has nothing to do with overheating- and you're right, the car isn't overheating. If there's a Carfax on it and somewhere along the line, the water pump was replaced, that would have been a good time to replace the coolant too. That may have been 100,000 miles ago though. If you simply drain and refill your coolant and the water pump isn't leaking the car will love you for it.

Coolant in an engine becomes acidic over time.  Premature failure of head gaskets. 

Flush it so you can sleep better.            

Have it tested.  Follow the owners' manual.