Regarding barbell rows (or anything that involves "bending over" under load) - it helps to think about keeping your back relatively straight and bending/hinging at the hips. The position of your spine shouldn't change much from standing to bent over. Push your hips backwards rather than bending over. And its perfectly fine to bend your knees. From the side, you should look like a 7 so the weight stays over your feet. This helps spread the load over you glutes and hamstrings rather than just your spinal erectors. If you look like an upside down L, back flat and legs vertical, then all the strain is going into your lower back.
It's good to work and strengthen your lower back though too. If it's weak, it's more prone to injury from everyday activities.