I'm using D3D11 for the Rendering API, and more relevant the DirectXMath header. Which i'm almost certain provides the helper functions for building, and manipulating quaternions for the sake of rotations.
presumably you are building a transform matrix from it all?
Yep, I have a simple setup so far. WorldMatrix is passed directly to the shader (simplifies for me at least a lot of calculations that require a worldspace transformation without having to undo a cpu side concatenation) and the view, and projection matrix I concatenate together, and send in separately.
I knew of quaternions, however, only in the realm of skeletal animation as using very little space, and being very efficient. The only reason I've never used them is due to me finding other means of mesh animation (vertex interpolation), and just not ever tackling them. But, i'll change that. Once I get the euler angle version working, i'll tackle the quanternion version.