If you were to shut down the engine at 12,000 ft and glide to landing, you'd save the 2 USG you'd have used for about 15 mins of SL cruise. So it seems you're right, it does appear to roughly add up!
But you don't shut down the engine, do you? You would run it at idle. My engines (IO-320) burn about 6 USG/hr producing about 90-100 HP, and 2 USG/hr producing no thrust at all.
There's an inefficiency in turning fuel into energy:
a) at low powers, where the engine is relatively inefficient
b) at high powers and low airspeeds, where a richer than optimum mixture is required for cooling
Hence I think there's some offsetting of fuel burn in the climb and descent, but on balance you use more fuel than flying level for the same time.