While doing the facial features, if we have to match our portrait, should our face pictures be used as a reference?
I would say yes. I've heard Kent saying that it is good practice to use the reference not directly in the viewport but on a separate window loaded in an image editor. Because if you have it in the viewport you can make the mistake of getting a flat result as you try to copy the face in orthographic view. Also having it just as reference to the side you begin to train your eye, which is super important, it's all about the eye.
In my experience, using photos as orthographic front and side modeling reference rarely turns out well. That's how I was taught to do it early in my career and my heads would end up boxy and not looking like the person despite facial landmarks aligning with the photos. It left me wondering "it's a perfect trace of the photos...why doesn't it look like the person??"
I've had more success creating a collage of many photos of my target person, from every angle you can find, and transposing that over to my sculpture by eye. Yes, it's a skill that takes time to develop but it's proven more reliable than overlaying photos to match.