A few thoughts...
First, "tithe" means "tenth"... ordinal (eighth, ninth, tenth) or fractional (one of ten, ten of 100, etc.). 5% is not a tithe, nor is 9.7%, or 10.2%, or 20%. The Law is specific; if people are going to argue in favour of tithing, they need to do so consistently with what the Law says.
Second, Abraham gave a tithe of war spoils before the Law, true. However, a regular tithe was not instituted under the Law. Further, Abraham was given the command of circumcision, which was also before the Law, and that was fulfilled in Christ, and is not binding on Christians. Therefore the argument that tithing predates the Law and is therefore still in effect is groundless.
Third, "tithing" is not "offering". The two concepts are distinct. Giving the tithe was mandatory, not optional.
Fourth, there are several tithes described in the OT... following the Law in this matter means following all the tithes. I have yet to hear any pro-tithing preacher tell people to buy wine or strong drink with their tithe, to enjoy in God's presence.
Fifth, in Genesis 28, Jacob made a conditional bargain with God. There is not a single verse of Scripture which suggests that he ever fulfilled his end of that bargain.
Lest anyone bring up the idea of tithing one's time, remember that such is two hours and 24 minutes out of every day.