Thanks - I did a bit more reading in the meantime, and I think I found the 'catch':
The 'catch' is that a December withholding of your Safe Harbor amount doesn't automatically protect you from a penalty, though it would in most/many cases. The wording I found is that a December withholding is treated as if it was spread equally across the 4 quarters. So...
Let's say, for easy math, your Safe Harbor amount is $4,000, and you had that w/h in DEC. That is treated as four equally quarterly payments of $1,000. Which will work in most cases, like if your other income was from dividends that were spread evenly across the 4 quarters. But...
If your income was 'lumpy', and much higher in one quarter, the even $1,000 might not cover that quarter. So a penalty could be applied if the deficit was enough to trigger a penalty.
So I should be fine, but in case I ever need to sell enough to generate a large cap gain, I could create a penalty situation for myself. I should watch that, and spread the gain across the 4 quarters evenly if I can.
It's never easy!