I'm not a fan of giving the government a year-long interest-free loan so I make sure I always owe a little at the end of the year
That's us too. When we both worked a single W2 job apiece it was pretty easy to do. The problem is now that my wife is full time self-employed and I have sporadic self-employment in addition to W2 it can be difficult to do this accurately throughout the year. For the amount of interest I'm losing I'd rather be owed $500 at the end of the year than be up worrying about penalties and interest for under estimating. I make heavy use of the "more withheld this year than owed last year" rule.
It's not so much that the self employment income itself is unpredictable. It is, but taxes are supposed to be paid as you go with estimated payments. It's the various deductions with income limits that can cause you to have way over or under estimated depending on your final numbers at the end of the year, which can be outside your control.
As an example, for the taxes just filed we lost about $800 on the child tax credit (not deduction, credit) all because one of my wife's clients that works on a strict Net 30 pay system accidentally settled a $9,000 bill way early. The invoice was sent around December 20th. In all the years she has worked for them she could expect payment around January 20th, the next tax year. Well due to a glitch in their expense system all vendors became pay immediately so the invoice was processed December 27th, the check cut December 28th and was in her hands on December 30th. It didn't help that I picked up $500 in side jobs in December, which would have been fine from a tax perspective if my wife didn't have an unexpected nine grand show up at the 11th hour.
I was able to blunt some of the impact by making a 2016 contribution to her IRA in January but was already halfway to her contribution limit.
Edit - How wonderful. My 1,000th post and it was on taxes.

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