(Let's do the prelimary 'background' thinggy first, eh?)
The very first Biblical references to tithing are from Genesis (14:20, 28:22) where we see it represented an act of reverence (by Abraham and Jacob respetively) in response to received blessing.
In the Mosaic Law, tithing was one of the ceremonial observances heavily tied to Israel's community life in relation to the Land they were promised (which in turn was related to the Levitical priesthood):
Let's get back to your question.
If you were asking, "Do I think that all Christians should observe
the practice of tithing as
specified in the Old Testament?" then the answer is obviously
No. With the change of priesthood and of law (Heb 7:12) due to the
work of Jesus, the OT tithing requirements lose their framework and thus
their relevance.
If you were asking, "Do I think that all Christians should give 10% of their monthly salary to the church each month?", then I'd respond with 1Cor 16:1-3, "Now about the collection for God's people:...On the first day of every week, each one of you should set aside a sum of money in keeping with his income, saving it up, so that when I come no collections will have to be made."
This verse offers some practical principles by which our giving may be demonstrated:
2Cor 9:6-11, "Remember this: Whoever sows sparingly will also reap sparingly, and whoever sows generously will also reap generously. Each man should give what he has decided in his heart to give, not reluctantly or under compulsion, for God loves a cheerful giver. And God is able to make all grace abound to you, so that in ALL things at ALL times, having ALL that you need, you will abound in EVERY good work."
This perfectly parallels the original spirit of the OT tithes, which was to teach and inspire the Israelites to love and be faithful to God (Deut 14:23, "Eat the tithe of your grain...in the presence of the Lord...so that you may learn to revere the Lord your God always.")
In conclusion: The modern Christian is admonished to give cheerfully (and excel in it - 2Cor 8:7!) out of the full and joyful awareness of the grace and generosity of God Himself. Our giving ought not to be sporadic but done on a regular basis; ought not to characterized by flippancy but should form an important pre-planned aspect of the Christian's budget; and ought to reflect a consistent proportion of his/her income (exactly what proportion is a separate issue, but in the OT people gave 10% and the early church gave a whole lot more - see Acts 2:45, 4:32! Food for thought, right?).
(Short commercial intrusion: A very detailed discussion on the relation between the Old and New Covenants plus all the key Scriptural passages can be found in 5 Views of Law & Gospel; an absolute mind-enricher indeed, *smile*).
2. What about churches which teach that we should tithe because that's what people did in the OT?
This teaching may not be 100% theologically correct (in my view) but a few things to note:
Hope this helps,
AL