I do not in particular care for Ikea and mostly just because of the store lay out. I don't go to a store for the entertainment value. I want to get in and get out.
With that behind me, I have never ever installed kitchen cabinets where everything worked like the drawings. Prefab cabinets should be built prior to install. And then units are normally attached to each other prior to attaching to the walls. You do not want to try to attach each individual cabinet to the final spot and then add another cabinet and so on.
And again, the wall being out of plumb should have had no bearing on ease of assembly.