Tax tables for filing single are significantly higher than filing jointly. I ran the numbers both ways and I find filing jointly works best for me. One of the problems is dividing up the deductions properly so the person in the higher bracket can get a tax break. One might also be subject to AMT sooner when filing taxes as a single. A married couple can do it either way and it only takes a couple of minutes to run some prelim numbers using the single tax tables for both.