Tithing is something that should have gone away. 
The reason why we don't keep the OT laws anymore is because we received a "New Deal" from Jesus. The only reason why pastors talk about tithing is because they need money to live - and do things with the church. However, if you give your time to the church and bring food to the church (pot luck, etc) then I don't see any reason to tith. With that said, your pastor will probably remind you that it's not your good works alone which get you into heaven... I guess money too?
Strongly related: Was tithing 10% required or encouraged by the early church.
As humorous as it sounds I recall that those who argue gross versus net refer to 'render unto Caesar what is Caesar's and God what is God's' - implying you pay gross to the government therefore you should pay gross to God.
Jesus said "Freely you have received; freely give." We aren't supposed to give out of an obligation. To display we are cheerfully givers, we give freely of our material things to God.
One correction: Priests, who were also Levites, were required to tithe: "Then the Lord spoke to Moses, saying: You shall speak to the Levites, saying: When you receive from the Israelites the tithe that I have given you from them for your portion, you shall set apart an offering from it to the Lord, a tithe of the tithe" (Numbers 18:25–2l
The Jewish Historian Alfred Edersheim seems to have done a fair bit of research on this subject. Basically, the subject is far more complex than a simple 10%, and would usually have amounted to more under Old Testament law. In fact, I am not sure I fully understand what Edersheim has explained—but that helps to convey just how complex it really was.
The basic idea of the tithe (i.e 10%) was bound up in the first fruit offerings of the land:
Two of these firstfruit offerings were public and national; viz. the first omer, on the second day of the Passover, and the wave-loaves at Pentecost. The other two kinds of ‘firstfruits’—or Reshith, ‘the first, the beginning’—were offered on the part of each family and of every individual who had possession in Israel, according to the Divine directions in Ex. 22:29; 23:19; 34:26; Numb. 15:20, 21; 18:12, 13; Deut. 18:4; and Deut. 26:2–11, where the ceremonial to be observed in the Sanctuary is also described. Authorities distinguish between the Biccurim (primitiva), or firstfruits offered in their natural state, and the Terumoth (primitae), brought not as raw products, but in a prepared state,—as flour, oil, wine, etc. The distinction is convenient, but not strictly correct, since the Terumoth also included vegetables and garden produce. (The Temple - Its Ministry and Services as they were at the time of Christ, by Alfred Edersheim (1825-1889), chap. 19, p. 379)
However, these first fruits and their offerings were complex in the traditional regulations and were largely based on being actually from the Holy Land:
They must be the produce of the Holy Land itself, in which, according to tradition, were included the ancient territories of Og and Sihon, as well as that part of Syria which David had subjugated. On the other hand, both the tithes and the Terumoth were also obligatory on Jews in Egypt, Babylon, Ammon, and Moab. The Biccurim were only presented in the Temple, and belonged to the priesthood there officiating at the time, while the Terumoth might be given to any priest in any part of the land. The Mishnah holds that, as according to Deut. 8:8 only the following seven were to be regarded as the produce of the Holy Land, from them alone Biccurim were due: viz. wheat, barley, grapes, figs, pomegranates, olives, and dates. If the distance of the offerer from Jerusalem was too great, the figs and grapes might be brought in as tithe.
I know the word "tithe" has "tenth" built into it, and so some Christians assume that every usage of the word "tithe" in Scripture is referring to "giving 10% of your gross income to the church." But I also know that "tithes" in the Old Testament did not refer to this at all.
They only "tithed" on food (livestock and crops); they had money (i.e. "income"), but the "tithes" did not apply to that.
The food went to the Levites, Priests, and needy people - none of whom had land of their own by which they could feed themselves.
"Tithes" were not required of everyone - only those who raised livestock or grew crops. For example, carpenters weren't required to tithe, nor were the Priests and needy people.
Clearly these things have application today; I am not challenging the idea of providing for ministers and needy people with cheerfulness and generosity.
My question is: When did the Church begin to understand "tithing" as "giving 10% of your gross income to the Church"? Clearly this is not what it meant in the Old Testament, and I don't see anything in the New Testament to indicate that the term was redefined by Jesus or the Apostles.
To clarify, I am not asking whether this is a good interpretation (or application, for that matter). I am also not looking for unsupported claims about how "it has always been understood this way, even by Jesus and the Apostles." My question is about Church History; please support your claim with references to historical records.
understand the commandment (principle & purpose) of tithing to the Levitical priesthood as defined in OT passages such as Leviticus 27, Numbers 18, etc.
But, my question is now (present day), when there is no temple or clear lineage of the sons of Aaron, what biblical proof texts do Christian (mostly Gentile) pastors use to claim that modern-day believers are commanded to tithe to them (namely, their non-profit organization) as obedience to God?
There are a couple of "proof" texts still used by pastors to command a tithe.
Bring ye all the tithes into the storehouse, that there may be meat in mine house, and prove me now herewith, saith the LORD of hosts, if I will not open you the windows of heaven, and pour you out a blessing, that there shall not be room enough to receive it. Mal. 3:10
Then there's the next question phrased do you want to be the head or the tail based on Deut 28? Pay your tithes or be the tail (cursed).
There are some groups that claim to be or hint at being of the Levitical priesthood and thus command their followers to tithe.
Lastly, there is the guilt trip.
But this I say, He which soweth sparingly shall reap also sparingly; and he which soweth bountifully shall reap also bountifully. 2 Cor. 9:6
To close, we are not under condemnation, but we give cheerfully.