I think the main principle is grouped under the head "information preservation" (or "information retention"): you don't throw away anything until you can prove some-one won't need it. (Being "conservative' in what you send means being conservative in what you thow away, in this case.)
This does not mean that you cannot *also* send the data in a converted format:
<width cm="33.33333">1/3m</width>
The recipient can choose which it likes. If in doubt, do both (if this won't cause integrity problems for later editing.)
In commercial publishing systems, the most common way to handle this situation, however, is neither to preserve the information as a fraction nor to convert it to decimal. It is to re-express the value as integers values against some very small common unit which is some reasonably accurate fraction of all the bases and demonimators desired. See EMUs (http://www.oreillynet.com/xml/blog/2007/04/what_is_an_emu.html) for example.
Cheers
Rick Jelliffe
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