Its 5 degrees into the LIVE engine that gives the best performance increase i think that the CAA only recomend wings level for aproaches when perpormance is not critical.
Part of the multi engine trainning will show you this as you should look at the Vmca with and without the bank into the live engine also try it into the dead engine and see the performance fall and Vmca rise (do this all at least 3000ft above the ground with an instructor)