Oblique interpolation and extrema points in DIN-style glyphs.

ok_locksmith_8414

Many sources suggest that when slanting glyphs for italics or obliques you should add extrema points to the outlines for various reasons. Often, this requires adding extra nodes. However, due to the design of my typeface, I face a couple of challenges. 

1. My typeface has a DIN like structure, so glyphs such as “O,” “o,” and “0” have flat sides instead of fully rounded curves.
2. My typeface contains both upright and italic styles in the same file, with the italics interpolated from the uprights via a slant axis. 

How would I approach this? 

I'm using Glyphs App by the way. 

SCarewe

I don't advise adding extreme points in slanted styles. Unless you find it easier to draw and manage shapes (which I highly doubt, given a bit of familiarisation, if necessary), I don't think there is any good reason to add them. Extrema in obliques only add problems: more difficult drawing (in my opinion), incompatibility, outline distortion (in comparison to the original slanted/corrected shape)...
The only objective reason I ever hear about adding extremes in italics is hinting. You can safely disregard this, unless you are doing manual hinting and know exactly why you are doing so. Hinting for italics is virtually impossible (again, unless you do it manually and know exactly what you're doing), and adding extremes would only help for vertical stems, which, by definition, don't exist in italics.
Bear in mind that fonts on Apple devices are hinted on the fly by the system, the font-internal hinting is disregarded. The same applies (to a lesser degree) on Android devices.