I moonlight in addition to my 9-5 job, and have had my tax software tell me to make esitmated payments in the past. I never actually did it, and never had to pay a penalty. IANAL, YMMV, etc, but are you going to make quarterly payments because the software told you to or because a real person who understands tax law told you to?

The way I do it, which is kind of like saving or making quarterly payments, is to overwithhold on my regular job. We still take the lifestyle hit of “saving” through overwithholding but don’t have the irritation of thinking about taxes more than once a year. When I estimate pretty well we get some money back and the software doesn’t complain, and when I don’t it tells me to make quarterly payments and I never have. It really makes cash flow easier, because I don’t have to worry about being on top of my billing or getting paid to make a tax payment.

So far, so good, for me. Might be worth talking to an accountant if you don’t like the idea of paying more than once a year.