Best modeling workflow for painting textures I know how to model, however, would it be easier to texture everything with a shape (cube, sphere, etc.) or with a plane that is mirrored? I haven't found any answers online.

Can someone please help me on which modeling technique would be best for texturing and materials? I love using the plane because you only have to worry about a couple vertices meanwhile if you model with a cube it would be doubled but for some reason I feel using a cube would be better with texturing because if something is mirrored it would look exactly the same on the opposite side, and sometimes I wouldn't want that.
If you are creating materials for others, it is a common convention to test them on a shader ball. See this question: Cycles advanced shader ball?

However, if you already have a mesh, it is best to test the material on it. The reason is that only the simplest materials are 100% mesh-independent, the rest have to be adjusted to the mesh, for example the depth of subsurface scattering depends on the scale of the mesh.