That is a great questions! I think "preferable" means to use best judgment. But I would remind you that space-saving is NOT a priority. Clarity and ease of reading comes first, along with accurate reflection of the print. My own preference is to start boxed material on a new braille page if it will not fit on the current braille page regardless of the resulting extra space at the bottom of the braille page. As a print reader, I prefer my boxes all on one page too! That is MY preference, but others may have a different preference especially in their own specific situation. This is why the guidelines say PREFERENCE. I think the old rules used to say that a braille box should be on a single page wherever POSSIBLE. And now it says PREFERENCE. Best judgment!
And of course in come cases a box may not fit on a single braille page anyway.