Vincent De Oliveira has written an epic post that details pretty much everything you might ever want to know about the
vertical-align properties. If you’ve ever had trouble aligning things next to text or wondered why two fonts look so wildly different from one another then this post is certainly for you.
You should always specify line-height as a unitless number (say this into the mirror five times). That way descendent elements that specify a different font size will inherit that number instead of a fixed line height.
Counterpoint in the comments by David Khourshid:
Unitless line heights are fine, but aren't the only way to use it. If I want an exact, perfect vertical rhythm implemented in my stylesheets, I would absolutely use a line-height *with a unit* everywhere I declare a font-size.
I'm a unitless man myself, only because perfect vertical rhythm always feels like a fool's errand to me.
Dave and I started the latest ShopTalk Show with an audio clip from Tim Brown responding to some of our previous chatter regarding vertical rhythm (and such). Transcription here. It sparked another interesting conversation about these things.
A small part of that was about Tim's coined phrase "molten leading", which is essentially
line-height that depends on line length.