If masonry fireplaces have one weakness in the South, or any wet climate, it is moisture. Bricks are naturally porous and are constantly battling the elements, especially alone above the roofline.
To preserve the life of your chimney it is very important to not just use a water repellent, but the right water repellent meant for application on a chimney.
I personally recommend ChimneySaver products. ChimneySaver provides great products for industry professionals that are backed up with with a strong warranty (they didn’t pay me to mention them btw).
Here are a few examples of what can happen as moisture builds up in your chimney:
It may look kinda neat but it is not good for the lifespan of your chimney!