Xjue,
Your assuming that you need more than idle thrust to taxy, depending on the aircraft type and weight.. Once the aircraft is moving it will happily go and even accelerate at idle thrust.
So you get a fuel saving by having less than all engines running and also a saving on brakes.
If it didnt work then the major airlines of the world would not be doing it!
Like everything else though, there is a time and place for everything and it should be trained and placed in the SOP's like when to start the second engine for example.
If your flying a bus then the manufacturer has already made these SOP's for your airline.
CaptJNS raises a good point, you will save fuel by using full climb thrust to cruise altitude - but depending on type you may be costing yourself more money on the engine costs.
To save money you need a top down approach in the airline across all departments, there are lots of factors that result in increased / decreased operating costs....