If you use pre-printed letterhead, it was designed to look best
when the text on the page has a certain amount of white space
around it. Heed your designer's advice, and stick to the margins
that were designed to be there.
For regular communications, use common sense. Leave enough white
space around the edges to allow someone to hold the page and not
block the text.
Cramming more information on any given page
just makes you look cheap or rushed or plain not able to plan
effectively. It reflects badly on your business. Make sure that
everyone who is responsible for corporate communications understands
this. If you are ever tempted to 'just squeeze it in', get a professional
to edit down the text. If your ideas are important enough to be
written down and communicated, give them the respect (and space)
they deserve.