You're probably OK to block the upper. But it depends what kind of driving you do. Don't take some guy on the Internet's word for it!
You need to watch your temp gauge and listen for the cooling fan.
Also how many miles on your car? Hope you've done your timing belt replacement if it's due (they tend to snap and wreck valves in Aveos after ~55k miles).
Feel free to add your car to the garage too:
Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com