Not all cages have plates parallel.
On the big ring they only need to be a few mm wider than the chain.
On smaller rings they need to be wider to allow for the chain moving more when shifting the rear mech.

I look from above; and
a) adjust the centreline of the cages to be parallel with the chainrings;
b) tweak the angle so that the smallest amount of trim is required when shifting the rear mech to its limits.
and
c) sometimes tweak the cage itself, especially at the nose of the inner plate.