Single channel texture sampling

Just wondering what the right way to handle single channel textures is in UE4 materials.

I understand that you should try to pack as many channels of data into as few textures as possible, e.g., use the alpha channel of the color map as the roughness input for materials that only have base color / roughness map inputs; pack the roughness / metallic / specular inputs as channels of a single texture, if you need them, instead of having 3 separate textures; etc.

But what about the cases where you have one input, e.g. specular, left over after you’ve packed all the other input data into as few textures as possible, or when you have a separate single channel map that can’t be packed together with other values, for example, because you are using different UVs for it, such as when doing parallax occlusion, etc?

Should I be using Sampler Type -> Grayscale for textures like that? (It seems no, since all the single channel textures I import into the editor are converted to color maps, that only work with Color samplers). If not, what is Sampler Type Grayscale meant for? What about Sampler Type Mask? Is there a resource you can point me to that describes the considerations that should be taken into account when deciding between different Sampler Types? Or should I just use Color for all regular samplers, irrespective of the number of channels, and the Grayscale / Mask samples are meant for some more specific use cases?