No red flag. You may incur a penalty for underpayment of estimated taxes. Rule of thumb is to have at least 90 % of your current taxes paid either through payroll withholding or 4 equal estimated payments. If your not sure what your current tax will be, you need to look at your previous years tax and to safeproof yourself from penalties you should paid in 100% of your prior years tax, or if you are a high income earner you should pay in 110 %. Use the 90% rule when your tax liability decreases from the previous year, use the 100 or 110 % when your tax liabilty increases from the previous year. But again not a red flag by not making estimated payments, it just may cost you a lot in penalties.