If you just want a normal map there is no need to texture the high poly model, indeed in most cases the high poly model doesn’t even need UVs. If you plan to bake out more than the normal map (using Kaldera or projection in 3dsmax) then of course you would texture your high res model first. Typically the normal map is generated for the low poly model before any texturing is done, it proves to be a useful aid in creating diffuse, specular and other maps as the texture artist can clearly see the UV layout and how it maps to the model.