This is ok, but beware of a IE5/Win bug:

"The nature of the discrepancy is that WinIE4/5 implements “small” as the initial value, rather than “medium” as called for in CSS-1. The result is that without correction, all sizes specified in keywords will be one size smaller with respect to user preferences in conformant browsers than in WinIE4/5."

See
http://www.alistapart.com/articles/sizematters/
for a workaround.

Actually, many web authors prefer "font-size keywords" since they never can get too small - which is possible with em or percent. And: ems and percents cascade, so you have to be careful constructing your CSS cascade.

Apart from that, IMO it is personal preference. With em and percent you can specify (relative) sizes in smaller units. If you don't need that, so what.

Another thing: If you specify the width of block level elements in em (I sometimes use that for the navigation bar in an all-out liquid design) then you might be more comfortable with specifying font-size in em es well, it is just more intuitive (at least for me).

Alex