So I figured this out on my side (I asked the original question). I claim 3 federal allowances in my W-4. 3 is good for me, I get more money in my paycheck, and I don't owe taxes in April. It balances out well based on my deductions and credits.
So when my wife filled out her W-4 last year, I told her to claim 3 as well. HOWEVER, her paycheck is smaller than mine (she works part time), and 3 allowances against her pay is enough to not pay any federal taxes.
dd_bills - I'm guessing 6 allowances against your wife's pay is higher than the amount she would have paid it taxes, so she paid none. My wife will be claiming 0 this year so we don't pay all of her federal tax when we do our taxes.
