VENTURA, Calif. — The portion of American households that tithe, or give one-tenth of their income, to their church dropped from 8 percent in 2001 to 3 percent in 2002, Barna Research Group reports. The Ventura, Calif.-based marketing research firm found that groups with the highest proportion of tithers were people ages 55 or older, college graduates, Republicans, Southerners, conservatives,…