I'm pretty sure that as long as you pay 100% of last year's tax, even if you have a very good year the next (which has happened to me for at least the past 2) you won't get hit with penalites.
For my state taxes I have to fill out 1/2 of their underpayment form (the easy parts), before hitting the magic line that says I owe no penalities or interest. Why I have to do this in a dozen lines when 1 saying "did you pay 100% of last year's owed" would do the trick is beyond me, but that's tax authors for ya! I then up the estimated for this year's to 100% of last. Only in bad years do I get a refund.
Yeah, I had decent capital gains last year, so had to go through all the rigmarole.
The state (NY) "underpayment of estimated tax" was a mild pain, because I didn't pay 4 equal installments (increased them slightly, figuring that I had some gains. This is, economically speaking, irrational, but I feared the spectre of a massive 4/16 payment. ). If you paid 4 equal installments, totaling at least last year's tax, it's an automatic and quick out. Otherwise you have to go through a worksheet. I wound up avoiding penalty, because each estimated payment was at least 1/4 of the required amount (last year's tax bill), but it was an annoyance. And I had to up my estimated payments a lot for this year. But capital gains means that I at least made some money, so I shouldn't complain too much...
And I had to do all my stuff early, in order to mail on Saturday, because we had a massive nor'easter which was expected to (and indeed did) shut everything down on Sun and Mon.