Make sure you keep an eye on inheritance. For example, if you set list items to 1.2em and have nested lists the nested list items will be larger than their parent list items.
Personally, unless I'm going to be setting box widths in ems, I set the font on the body to 75% (12px in most Windows and Mac browsers) so that text is readable without having to have multiple settings (10px is very often far too small for most people, and so a setting over 62.5% generally needs to be overridden elsewhere in the CSS).
Saw this on a mailing list I am on - I wasn't aware of it, and thought i'd share!
If you want good font size control, set the font size of your body to 62.5%
This should effectively give you a starting size of 10px. Then you can
use the magic of ems to scale your fonts with pixel perfection.
1.6em gives you 16px font.
1.2em gives you 12px font and so on.
Even though you have font sizing to the pixel, it's still able to be
changed by the browser as desired.
