Most of the Christian fiction books I've read are Young Adult novels, so there would be limited foul language regardless. In one series I read there was no cussing and it worked. The author managed to bring them to life without it. That said, I read another set of books and each of them was horrendously self-righteous and so 'goody-goody' fake I could barely finish the books. I felt like a horrible Christian afterwards because I'm not as perfect as every Christian character in the books (only the non-Christians had imperfections or committed sins).
Make your characters real. The fact is Christians are not perfect. We sin, we make mistakes, we are as human as everyone else. What makes us different is our belief in Jesus and in his sacrifice to cleanse us of our sins. Expressing frustration or anger through words is not a sin. The closest sin to cussing would be taking the Lord's name in vain.
A word is simply a word until we add our expression, intention, and emotion into it. I often say the word crud if I'm angry or upset by something, thinking it's better than saying a cuss word. Well, my daughter repeated it the other day with the same type of expression and I realized it was just as bad as if she'd been saying s**t.