Sorry, I have this conversation with my graphic artist a lot
My standards, in order of preference:
1) Use percents or ratios. Print layout people are trained very well to override this with a ruler, but it is the natural layout vocabulary. If you look at a newspaper, most people immediately recognize that it has two, three, or four columns, or that the ads are 1/4 page. They don't know (and don't care) that 1/2 of the page equals 6.32 inches. Since we have techniques built into CSS to do this math automatically, use them.
2) Use relative length units. To quote CSS2:
There are two types of length units: relative and absolute. Relative length units specify a length relative to another length property. Style sheets that use relative units will more easily scale from one medium to another (e.g., from a computer display to a laser printer).
Relative units are:
em: the 'font-size' of the relevant font
ex: the 'x-height' of the relevant font
px: pixels, relative to the viewing device
These are to be preferred when the primary content is textual, the most common case for Web documents. Since you are already defining your font size, whether in relative or absolute units (and I prefer em's), you chain the rest of your display (which usually bounds that textual content) to those sizes by keeping things relative.
3) Absolute units: again a quote from CSS2:
Absolute length units are only useful when the physical properties of the output medium are known. The absolute units are:
in: inches -- 1 inch is equal to 2.54 centimeters.
cm: centimeters
mm: millimeters
pt: points -- the points used by CSS2 are equal to 1/72th of an inch.
pc: picas -- 1 pica is equal to 12 points.
'Nuff said. I only use absolute units on "@media print" sections.
Not only does the above prioritization allow you to scale better on a vast range of clients, but it allows the user to make small changes (like enlarging the font size using a browser preference) with global, yet unconflicting effects.
