When you have a major change to your tax situation --think getting a raise, getting married, wife going back to work, having a kid-- you need to adjust your withholdings. I think it is a good idea to check to that you are having the right amount withheld at the beginning of each year, and then to check again in the summer. This will allow you to prepare for your tax situation. The IRS just updated the withholdings calculator for the 2013 tax year. It is a good idea to go and check it out. You can plug in your current earning, and get a solid estimate on what you are going to need to have withheld in taxes.
Just a word of caution, if you are self employed, the online calculator does not calculate your self employment taxes. You will need to figure out the formula yourself or talk to an accountant to make sure you are making the right amount for your quarterly tax payments. You do not want to end up owing a lot of money at the end of the year. If you are self employed, do not forget that you will need to pay quarterly sate taxes as well.