Lumbar flexion is safe. Even under loaded conditions. You can’t avoid lumbar flexion while performing squats or deadlifts. Living tissues adapt to load. Cadaveric tissues do not adapt to load (***cough cough*** Stu McGill).
The ideal lifting form is the form that allows you to:
Maximize performance (You probably aren’t going to perform your best when your body is like a limp noodle)
Do you feel in control of the weight? (How do you know if you are in control of the weight? This is what warm-up sets are for.)
For more information, please refer to this article by Greg Lehman