Here's a short discussion on the differences:
CSS Font-Size: em vs. px vs. pt vs. percent
There is some benefit to working in percent, but as long as you set the font size in % in the <body> then ALL other (font-size) references can be made in EM to keep them relative to each other regardless of screen size.
eg:
Code:
body {font-size:100%}
p {font-size:1em}
h1 {font-size:1.5em}
h2 {font-size:1.3em}
h3 {font-size:1.2em}
h4 {font-size:1.1em}