JOEL - Normally, church members who call themselves "full tithe payers" pay 10 percent on the Gross income of their salaries along with any other sources of money they might receive throughout the year. The law of tithing has ancient origins. The word "tithe" means "tenth" and connotes a tenth part of something given as a voluntary contribution.
Joseph Smith received a revelation about tithing from God which states:

“And this shall be the beginning of the tithing of my people.
And after that, those who have thus been tithed shall pay one-tenth of all their interest annually; and this shall be a standing law unto them forever, for my holy priesthood, saith the Lord.” (D&C 119:1-4.)

The phrase, "all their interest annually", has been interpreted by church leaders to mean 10 percent of all the money you earn in your paycheck(Gross) or receive by other means. However, the exact dollar amount that a person decides is a full tithe, is really left to their own discression of what they feel is an "honest" tenth.

Here are some statements from past church leaders:

Marion G. Romney, an apostle of our Church once said;
"My plea this day to all members of the Church is: Pay an honest tithing and be blessed, and don’t quibble over the amount you should pay."

President Heber J. Grant;
“I have found a great many people who do not know what their tithing is. I have never met people of that kind but what I believe if I were in partnership with them and they had a tenth interest in that partnership, they would know pretty well what that tenth was. I do not think they would have any difficulty whatever in finding how much I owed them. So I am inclined to think that if we wanted to, we would have no difficulty in finding out what is one-tenth of our income, and that is what we owe to the Lord.” (Improvement Era, Jan. 1941.)

President Wilford Woodruff;
“I want the brethren to understand this one thing, that our tithing, our labor, our works are not for the exaltation of the Almighty, but they are for us” (Emerson Roy West, Profiles of the Presidents [Salt Lake City: Deseret Book Co., 1972], p. 155).

President Lorenzo Snow;
“A part of a tithing is no tithing at all in the eyes of the law that the Lord has revealed” (Life of Joseph F. Smith, comp. Joseph Fielding Smith [Salt Lake City: Deseret Book Co., 1938], p. 308).

President Joseph F. Smith;
“The law of tithing is a test by which the people as individuals shall be proved. Any man who fails to observe this principle shall be known as a man who is indifferent to the welfare of Zion, who neglects his duty as a member of the Church” (Gospel Doctrine [Salt Lake City: Deseret Book Co., 1939], p. 226).

President Spencer W. Kimball;
“Inquiries are received at the office of the First Presidency from time to time from officers and members of the Church asking for information as to what is considered a proper tithe.
“We have uniformly replied that the simplest statement we know of is the statement of the Lord himself, namely, that the members of the Church should pay ‘one-tenth of all their interest annually’ which is understood to mean income (see D&C 119:4)” (in Conference Report, Oct. 1980, 113; or Ensign, Nov. 1980, 77).

President Gordon B. Hinckley;
"Tithing is a principle that is fundamental to the personal happiness and well-being of the Church members worldwide, both rich and poor. Tithing is a principle of sacrifice and a key to the opening of the windows of heaven. In Primary I memorized the tithing poem: “What is tithing? I will tell you every time. Ten cents from a dollar, and a penny from a dime."
"For many years, presidents of the Church have interpreted “interest”(D&C 119:4) as “income.” Beyond that, they have not elaborated. That fourth verse consists of thirty-five words. Contrast that with the cumbersome and complex tax codes enacted and enforced by governments."(Ensign, Dec. 1989, 5)

As you can see, most of the statements I could find don't really specify whether we should pay on the Gross or on the Net.
A 1970 letter from the First Presidency of the LDS Church stated that, notwithstanding the fact that members should pay one-tenth of their income, "every member of the Church is entitled to make his own decision as to what he thinks he owes the Lord and to make payment accordingly" (Mar. 19, 1970; Doxey, Roy W. Tithing: The Lord's Law. Salt Lake City, 1976).
Hence, the exact amount paid is not as important as that each member feels that he or she has paid an honest tenth. We are promised blessings if we do pay an honest tithing(Mal. 3:8-10). Therefore if a person is trying to decide on whether he should pay on the Net or on the Gross, he should ask himself this question; Do I want Gross blessings or do I want Net blessings? :-)