There are a lot of opinions about this and I suppose the best answer is that you can't declare yourself to be a full tithe payer unless you are current when you attend tithing settlement. However, I did a search at LDS.org for the phrase "tithing first" and got 39 hits. Here are excerpts from 13 of them.
1. Advice from a member of the First Quorum of the Seventy:
"We pay our tithing first and then the Lord helps us to use our nine-tenths wisely. I can bear testimony of that." (Derek A. Cuthbert, New Era, Jul 1983, 46)
2. Advice from a former member of the Seventy:
"I remember attending tithing settlement as a youngster with my mother and father. It was such a natural thing to me to visit with the bishop and to declare myself a full-tithe payer. Even as I got older and started earning more money, I always paid tithing first....
"When Sister Richards and I were first married, we were going to school and had very little income to meet our expenses. Paying our tithing was a great sacrifice. But Sister Richards never even considered using our tithing money for other things we greatly needed, like food or rent. She insisted that we pay our tithing first, and we always did." (H. Bryan Richards, New Era, April 2007, p. 42.)
3. From a Family Home Evening lesson:
"Pay your tithing first, and you will be blessed in helping your family.... Ask family members to list the potential blessings this family could enjoy by paying their tithing first." ("Tithing," Family Home Evening Resource Book, p. 227.)
4. From a Sunday School lesson:
"'From my grandfather I learned early in life that you give only your best to the Lord ' (quoted by Dallin H. Oaks, in Conference Report, Apr. 1994, 46; or Ensign, May 1994, 35)....
"Now that we usually pay tithing with money, how can we give our very best to the Lord? (Answers may include that we can pay with the proper attitude and pay tithing first, before we use our money to buy things we need or want.)" ("Lesson 29: Paying Tithing with the Right Attitude," Preparing for Exaltation: Teacher’s Manual, 169.)
5. How to get out of debt and stay out:
"Pay tithing first" ("Taming the Debt Dragon," Ensign, Oct 2004, 50–53
6. From a Primary lesson:
"Using imitation money (you can make your own), have the children pretend to be a family and make a budget for the amount of money you give them. Have them budget for food, rent, utilities, clothing, and recreation. Then tell them to add tithing and other offerings to their budget by placing them first on the list. Explain to the children that we should always pay our tithing first because that is the Lord’s tenth." ("Lesson 24: The Widow’s Mite," Primary 7: New Testament, 78)
7. Advice from a bankruptcy attorney:
"The most important step toward achieving financial well-being is to pay tithing first—no excuses and no exceptions." (Lane V. Erickson, "Five Steps to Financial Well-Being," Ensign, Mar 2004, 66–71)
8. From an Aaronic Priesthood lesson:
"What did President McKay’s father teach him? (When we love our Father in Heaven, we want to give him our very best.)
"How can we apply this principle to how we pay our tithing? (We should pay our tithing first and not wait and pay out of what we have left at the end of the month.)" ("Lesson 16: Tithes and Offerings," Aaronic Priesthood Manual 2, 56)
9. Advice from a member of the Presidency of the Seventy:
"Pay your tithing first, and avoid debt insofar as possible. Remember that spending fifty dollars a month less than you receive equals happiness and spending fifty more equals misery." (Joe J. Christensen, "Marriage and the Great Plan of Happiness," Ensign, May 1995, 64)
10. The first tip for financial success:
"Pay your tithing first." (Kristi Linton, "Money Matter$," New Era, Sep 2004, 34)
11. From an Aaronic Priesthood lesson:
"Ask the young men how the individuals in the following situations could seek forgiveness and be prepared to partake of the sacrament.
"You have earned some extra money by doing yard work and have used that money to buy a new bicycle. During sacrament meeting, you realize that you have spent all of your money and have forgotten to pay your tithing. You do not feel right about this.
"What could you do? (Pray silently and ask for forgiveness; covenant and promise to pay the overlooked tithing from the next money you receive; covenant to always pay tithing first.)" ("Lesson 35: The Sacrament," Aaronic Priesthood Manual 1, 123)
12. From a Family Home Evening lesson:
"Tithing was quickly agreed upon as the most important thing we have promised to pay. We have always paid our tithing first." ("Money Management," Family Home Evening Resource Book, 210)
13. Advice from a new convert:
"Acting on faith, we committed to paying tithing first. From that day forward we were always able to put food on the table and make ends meet." (Lorin John Hamilton, Ensign, Sep 1999, 29)