Effect of Personal Allowances
Each personal allowance you claim reduces the amount of income subject to income tax withholding. Use the worksheets that come with Form W-4 to figure out the maximum number of exemptions you're entitled to claim. However, there is no minimum number of allowances you're required to claim, so you can still claim "0" if you're married.
Effect of Claiming Fewer Allowances
When you claim fewer allowances than you're entitled, you'll end up having excess taxes withheld from your paycheck during the year. That means you'll get extra back when you file your income tax return. Some people prefer to get a big refund as a way of forced savings. However, the IRS won't pay you any interest on the excess withholding, so by having extra withheld, you're essentially making an interest-free loan to Uncle Sam.
When you're married, you also have the option to check a box to have money withheld at the higher single rate. The IRS has different tax brackets for married filers than it does for single taxpayers. If you are married, you'll have less withheld than if you're single because the tax brackets for married couples are larger. For example, for the 2015 tax year, if you're single you pay a 10 percent rate on your first $9,225 and then 15 percent on the next $28,225 of income. But, if you're married filing jointly, you pay that 10 percent rate on your first $19,450 of income and 15 percent on the next $56,450.
Reasons to Claim Single Withholding
According to IRS Publication 505, sometimes married taxpayers don't have enough withheld from their paychecks, especially when both spouses work. You may be able to solve this by opting to have taxes withheld at the higher single rate. However, if both spouses work, you may also be able to solve this issue by completing the Two-Earners/Multiple Jobs Worksheet instead of using the Personal Allowances Worksheet. With this worksheet, you can figure out how many allowances to claim for your highest-paying job and then claim "0" allowances on your remaining W-4s.