Should I learn it early on (like on the early stages of a first game)? Or later (like mastered the DX fixed functions and have created some games)? Should I learn the assembly like way first then HLSL, or HLSL first then the assembly like way?The challenge in graphics is understanding the math going on behind the scenes, not mastering any given API, whether DX, OGL, HLSL etc. Using shaders, or GPU acceleration in general, is just a matter of getting the same things done faster. The advice I have been given by my professor is to do everything on the CPU while you are figuring it out - keeping things simple and nice to debug - and move it to the GPU later if and when you actually need performance.
In short, I think messing with shaders is quite likely going too far into technical detail for a first game.