Unitless or not unitless?
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.