We fly A319 to A321 and are encouraged to shut down No.2 if longer taxi-times are encounterd (i.e. waiting for the gate to be cleared) up to CPTs judgement.
During normal taxi-in (even during a longer one) it's rarely done, to much procedure, switching etc. and the a/c handling is far better on two, especially when heavy and you have to stop rather frequently.
Engine FF ist about 280 kg/hr in IDLE, the APU burns on ELEC only about 80 kg/h so for a 5 Minute-Taxi you could save some 15 kg. Theoretically. Maybe.
Regards, MAX (anti-beancounter)