The PROBLEM with that is for many of us, we do not know the total amount of credits/deductions we are going to have at the end of the year, nor our total income.
For example, I will not know how much I spend on child care until after the summer is over. I won't know the total interest paid on student loans until the end of the year. I won't know total tuition paid etc..
Heck I won't even KNOW our total income until I get those w-2's (since BOTH Dh and my incomes vary greatly on bonus income each year).
I've never had each year be anywhere the same as the previous, thus planning what we are going to get back doesn't work. Last year my total tax due was almost $3,000 less then this years, but this year I got twice a large of a refund (due to credits and such).
My DH has almost very little taxes taken out, but he also has almost all the deductions to income (insurance, flex accounts, 401k). I just have my 401k deducted. I have my set at either 0 or 1, but I also make way more money then my dh does (which means MY income is the bulk of what we pay taxes on). This way, if we do go short (and we have in the past) its not going to be a large blow.
BTW we usually end up breaking even or owing on state (anywhere from $100-$500 owed). The only year we have ever owed for federal was the first year I was back working after grad school (because my income shot up suddenly and I didn't have full interest on loans and such)
The problem with not knowing our total income is that we can easily fall right over a line pushing us into a new bracket suddenly at the end of the year and thus our taxes will be higher (and even get pushed into AMT at some point but that is a whole other fear)
Don't look at your witholdings in a vacuum, you need to look at them via how much you make and how much DH makes. It doesn't make much sense to have the person making the most money also have all the deductions and the most taxes taken out of the lower paycheck (unless they are close in pay). If we did that, we would end up owing thousands of dollars each year (because we wouldn't have the all the tax money I paid in to cover our taxes due)