Let's be clear here. You can use the 90C column as the starting point for derating, presuming the conductors have 90C rated insulation. If for example you plan to run more than three current-carrying conductors in the same conduit. But you cannot just take the value in the 90C column and base your installation on that value.
Conductors from a transformer secondary to switchgear, is actually one place where you may be able to use the 90°C rating. These conductors often land on bus bar at that switchgear that does not have the 75C° limit. Same with some transformers, no 75°C limit.