This is a perennial tax question. Usually filing jointly saves a couple taxes. And joint filing is required in order to use certain tax breaks. However, there are instances where filing separately makes sense. Strategy: Figure the taxes both ways (joint/separate) and choose the option that saves you and your spouse the most money. But money aside, if you are concerned about the tax liability of your spouse, separate filing avoids joint and several liability for the taxes on a joint return.
Generally, an unmarried person who maintains a household for dependents and is allowed to compute his or her tax based on head of household rates, which are more favorable than single person rates.