12V / 5A is regarded as the average sufficient enough value for a reprap without a heated bed.
I'm not sure you'd be making a real save on anything by using something smaller, and "getting away with" might be something that fluctuates depending on weather conditions, type of object you print etc.
I mean keeping a margin is better. But feel free to experiment
Most of my technical comments should be correct, but is THIS one ?
Anyway, as a rule of thumb, always double check what people write.