Since announcing our intention to pay off our mortgage in six years, I've been asked if that's wise, given that the interest on a mortgage is tax deductible. The short answer is, it doesn't really matter to me. Paying off the mortgage will free up a lot of cash flow, which in turn will open up lots of options for us - options that I most definitely want to have in six years, because they may include things like the very expensive private school that we can't afford right now.
The long answer is - no surprise here - it depends - on whether you can earn more money with the extra principal payment than you would save by paying off that amount of principal. The math gets kind of complicated (at least for me), and I'm not a CPA or tax lawyer, so let me know if I haven't got this right.
Let's say that you'll pay $10,000 in mortgage interest this year and are in the 25% tax bracket, meaning that you'll pay $2500 less in taxes thanks to your mortgage. Let's also assume that if you pay an extra $250 per month in principal, you'll reduce your mortgage interest to $9,000, and therefore reduce your tax deduction to $2250. In other words, you'll end up paying $250 more in taxes this year because you paid off $3000 extra in principal ($250 x 12) . But you also paid $1,000 less in mortgage interest, for a net gain of $750.
Now, suppose you could take that $250 per month and invest it somewhere and earn more than $750 over the course of the year (plus more, to cover the taxes you'll owe on the extra income). Then theoretically, you're better off not paying extra on the mortgage but going with the investment instead. However, it also occurs to me that the math ought to be even more complicated because the mortgage interest savings are probably compounded over the life of the loan - that sort of calculation is way beyond my abilities, so I'm not even going to attempt to go there.
In my case, I haven't run the actual numbers to see if we could earn more via investment than we'll save by paying off the mortgage early. I don't even want to attempt those calculations, and in any event, it doesn't matter because I would predict the gain wouldn't outweigh the options that I'm looking forward to having in six years. And for us, that's what really matters.