Through the years I have heard to press the lower back to the mat and then I hear that it is better to keep the back neutral. I had heard that keeping the back neutral creates more of a challenge and we work more of the ab muscles.
This is relative to assessment and goals. It is always better to maintain a neutral spine but for some that might mean they have to press the back into the mat to realign from poor posture. As is everything in training what we program is directly dependent on what we uncover in assessments.