Performing some lower ab exercises as part of your regular strength training routine is a very smart move. This section of the body is largely responsible for maintaining the alignment of your back and preventing lower back pain. Doing a lower ab exercise that teaches you to maintain focus on this portion of your back is going to be your best bet because most people are not even aware of whether or not they are maintaining proper posture and if they are using their ab muscles to control this part of the body.
The best lower ab exercise to perform for this is simply lying on the floor, back down. Then, bend the knees so they are slightly bent and your feet are flat on the floor, arms down by your sides. After doing this, simply pretend that there is a string that is pulling your back through the floor. While thinking that, simultaneously think of trying to press the lower back into the floor as much as possible by squeezing the bottom portion of this muscle. If you stick your hand under your back by this area it should feel completely tight, with no space between it and the floor. Hold this position for 30 seconds or so and then relax. Remember to breath while you are doing it but think about moving your lungs up and down rather than straight out. This will also teach you the proper breathing technique that you should be using while exercising or just in general (breathing in this way tends to lift the body and improve the posture).
So make sure you do not neglect this aspect of the body with your training. While it may be a small muscle, the bottom stomach muscles are essential for proper functioning of the body.