In a recent project the countertop contractor digitally measured the walls, even ones that look square weren't, not a 90 degree corner to be found! You should build cabinets as squared boxes and level them when installing. The trim work that surrounds it will hide gaps, etc.

Generally if there is space because of odd angled walls there really isn't any advantage to trying to use it. You're talking square inches of drawer/cabinet space, it really doesn't matter as far as usable space is concerned. Its a huge hassle to try and build cabinets that are not square - things like how do you screw/nail parts and not have the screw/nail blow out a finished side? It can be done, but should it?

I have not opened up that can of worms yet, but have been thinking more about tapers and other non-rectilinear forms. Longer-term I very much want to support a "fill this space" mode for built-ins.