As for single engine taxi I often hear from people that yea but if you taxi uphill to the gate area then you have lost the benefit by using higher thrust setting on running engine". I am no tech geek and I am not sure if the statement is correct, but, if you think this is the case - Don't shut down the engine!! But shut down the engine in all the other 95% of you arrivals and you will, over time, save money.... How hard can it be?
CaptainProp: Some information for you. For a CFM 56-5B5 one engine at roughly 30% N1 produces a greater amount of thrust and burns LESS fuel than two engines at 19% N1. The arguement about upslopes needs to kept in context as it most probably forms only a small part of the total taxi time and the fuel flow, measured by the ECAM, isn't calibrated to that degree of accuracy.
My Company have published a breakeven distance (i.e., cost) to allow us to make an informed decision about additional taxi time vs. CONF 3 - it gives us a number to work from for RWY exit planning etc.