The income tax system is supposed to be pay as you go, and she is required to make estimated tax payments on a quarterly basis. For income earned from January to March, the payment is due April 15. For income earned in April and May, the payment is due June 15. For income earned June through August, the payment is due September 15. And for income earned September through December, the payment is due January 15. (These are the right dates even though they are not exact quarters.)
Although there is a technical requirement to pay estimated payments, no penalty will be assessed as long as the total tax paid into the system for you and your wife on your joint return meets the conditions outlined by @Mike9241 . You can pay into the system by making estimated payments at the IRS web site, or by having more tax withheld from your W-2 jobs. The IRS doesn't care how you pay as long as you pay. If you over-pay your estimated taxes, the excess will be refunded when you file your tax return.
If you owe more than those amounts, you can be assessed a penalty for underpayment even if you pay in full when you file your tax return.
