The element that governs your whole layout
is the size of your images.
If you have large (or many) graphics then
you're forced to use pixel widths to control
If you have few, or small graphics then you have
much more leeway to use whichever method you
feel like (like me). Using percentages means that
your pages will ALWAYS appear as you intended,
no matter what resolution your users' screens are at.
Alas, designers tend to cram more and more
graphics into their pages, which is why percentage
widths are becoming rare.
I'm not saying what's right or wrong, this is just
my own opinion. I don't usually talk about my design
preferences here because somebody always disagrees, but
I couldn't resist this one :)