I don't believe we need official guidelines for this - any more than
we need official guidelines about how <em> should be rendered.
Possibly suggestions, but that's about it.
On the other hand, UNOFFICIAL guidelines would be more than welcome,
and could be written by anyone interested. If overviews of the
available printing software (as opposed to browsers, which I find do a
miserable job of printing HTML) were attached and kept up to date, it
would be nearly invaluable.
As a final note, I think the style-sheet mechanism should include - at
the very least - places for "style" items for media other than a
display. For example, setting widow/orphans for printing would be
appropriate for a style sheet.