lucy24 - 7:51 am on Aug 1, 2013 (gmt 0)
What are the alternatives? How many side-by-side subdivisions are you using?
In My Personal Opinion ... almost anything in the universe is better than using pixels. But the choice of percentage vs. em depends what the page looks like. Don't let your percentages add up to exactly 100. Leave a little wiggle room.
Come back in a few years and you'll have more options, because CSS3 incorporates computed values ("10% plus one em", "four ems plus 3 pixels", "half of whatever is left over").
Here as elsewhere, it often works out better if you first figure out what you want to do, and then develop the best code to do it.