1. As my mom would always say, "Telling one lie just leads to another and another and another."
Why waste time in an endless loop of lies? I've seen this many times as an employee and as an employer. When a person tells a lie or a partial truth (same thing), it usually leads to more of the same. After a while, they can't even remember what lies they told.
2. All it takes is one lie to lose a client's trust
Good clients are hard to find sometimes. Lying is one of the surest ways to lose them. I've gained most of my clients because someone else wasn't honest in their dealings and couldn't be trusted.
3. You can sleep peacefully at night
It's so refreshing to know that you haven't lead anyone astray (including the IRS), and there is no one coming after you because of something you mislead them on. Even if you have consequences or losses due to telling the truth, peace of mind far outweighs any of that.
4. If you lie in business, you'll probably lie in other areas
Once you make something a habit or an acceptable practice, it inevitably spills over into other areas of your life. When you lie about completing work for clients, you probably have no problem telling your spouse that you took care of something that you didn't. Or, if you withhold taxes in your business, you'll probably do the same in your personal income taxes.
5. It's a commandment
This is, by far, the most important reason for me. Even if I was able to get away with telling an untruth without anyone finding out, I know that God is watching me and what He says about the situation is much more important than the opinions of man.