Umm, no. You can technically engage engine braking at almost any speed down to 10-20mph.Its kinda an oxymoron because if you drive in high enough RPM to engine brake your driving at too high of RPMs to get good normal mileage.
Fuel economy is best if you keep speed pretty much consistent... say cruising 50-60mph is the typical sweet spot for most.. not too slow that you can't shift to the highest gear and not too fast that wind resistance starts to become a drag (no pun intended).
You can engage the engine brake technically by just removing your foot from the gas pedal and coast the rest of the way, like when approaching slow moving traffic or a stop. Coasting in gear does not use any gas at all (if the engine RPM remains above 1k) as the car's momentum is keeping the engine from stalling. Also particularly useful on long down-hill runs. Just be careful you're in the right gear for the right speed (M/T) so you don't over-rev the engine.