It depends on who maintains your engines / apu's.
The effect of 2-3 degrees reduction in flex on the engines depends on how much flex is being used.
i.e. at close to TOGA a reduction in 2-3 degrees is very significant as regards to engine wear. Not so much at lower power settings.
At my company it probably costs just as much using the APU as we do all of our take-offs with the packs off anyway.
FIS.