Thanks everyone. Sounds like the consensus is the quarterly payments do not have to be equal, as long as I don't underpay my taxes. If I do underpay, then I can avoid penalty by filing form 2210? Is that right?
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The basic idea is that IRS wants payments to be based on when your income is received. The default IRS assumption is that your income is received evenly during the yr. Hence the expectation of equal quarterly installments.
If you pay in non-equal payments, you will have to demonstrate via F2210 that you paid according to how your income was received. This works if you received the bulk of your income in Q4 and the major payment was also in Q4.
If your income was received evenly during the yr and you paid in non-equal payments with a big payment in Q4, F2210 won't help you and you may be penalized for late payments for the earlier quarters.