I have heard more than once that if you are in a long-term relationship, it's ok if you fight; in fact, it is bad if you don't. Well, I would have to disagree. My boyfriend and I don't fight, but we also don't keep things from each other. We don't fight because we have really really good communication skills. Instead of keeping things from each other and letting them build up inside us waiting to explode into an ugly fight, we talk things out when we have something difficult to discuss.
My boyfriend and I have only had one fight in our more than three and a half years of dating, and it was a stupid and unnecessary fight. It most likely wouldn't have even happened if we had better communications skills during it. I can't imagine what it would be like if my boyfriend and I ever got into a serious fight. I couldn't live with myself knowing he was angry at me or if I was at him.
Just because we don't argue, it doesn't mean we keep things from each other and we don't tell each other what the other needs to hear. There have definitely been those moments in our relationship. But we are at a point in our relationship where we are comfortable with doing that for each other because we know in the end, it will benefit the other person.
So if you are in a long-term relationship, but you don't fight, don't worry. As long as you have good commutations skills, your relationship is a-ok. But every relationship is different, so if you don't fight, but don't have good communications skills, you may need to re-evaluate the way you communicate. It is completely up to you. Just make sure you do what's best for you and your partner.