Speaking strictly for myself... this is basically why, despite having grown up in a metric country and technically living in one (Canada) I still sometimes use

Speaking strictly for myself... this is basically why, despite having grown up in a metric country and technically living in one (Canada) I still sometimes use inch, foot, ounce and sometimes even gallons for rough estimates. If the unit fits the quantity, the uncertainty is implied as in "he was about six feet tall" or "add a couple of inches".