Google IRS Safeharbor Rules This will give you the authoritative answer to your question. There are specific rules in place that allow you to pay based on prior tax year numbers that are knowns. If you follow these rules you can avoid penalties no matter what your income ends up being at the end of the year.

Personally I’m not in agreement with the “just spend it” crowd, or the “just ignore it” crowd. I’m more of a spend less, save more and “knowledge is power” kinda guy.