Depends...on your design and layout. The second number declares the line-height of the font. If you increase the font-size to 14px from 12px, the line-height will remain at 1.4em making for text that has its lines closer together.
But that's why most of us don't use units on the second number, so the second number would/could/should be '1.4' and used as a multiplier of the font size. So the line-height would then become 14px times 1.4 and adjust itself as the font size changes.
Whether that fits the rest of your page is up to you.
To tell the truth, I've always used the second number as a multiplier and I just don't recall whether it is valid to use units, such as 1.4em. But I have to leave so I'll leave that as an exercise for the reader. :)