For the stuff I write that's work-related I leave margins wide enough for marginalia and footnotes; and also work single-sided. Those measures leave me lots of room so that I can add stuff later on as needs be, hence avoid 'tipping in' pages which is frowned upon.
For personal writing, I make an effort to leave margins wide enough for comfortable handling of the sheet, so that if I happen to be using a less than robust ink it will not be smudged nor transferred to the readers' fingers; and avoid hyphenation. (I doubt that anyone adds marginalia etc. to my correspondence.)
__ EDIT to add: Kindly consider what I call the 'tyranny of the page'. To give me a blank bit of paper is just a start - a landscape sheet from an A4 can evolve to an A3. Lots of room to roam. I welcome a blank A0; and more than a few of my friends have received personal letters written on a few such large sheets, rather than a sheaf of A4s. No origami was involved, just the ISO standard for folding. Perhaps the recipients thought, "Crikey mate, what's wrong with a post card? We don't need to paper the ceiling"
Edited by Sandy1, 11 June 2016 - 11:14.