Two days before our scheduled cleanout, my cousin found a shoebox marked "TAXES 1996" in the back of a closet we'd already cleared for donation pickup. It wasn't taxes. It was almost three years of letters between my grandmother and her sister, who she hadn't seen in person since 1961.
After that, we changed our whole approach: nothing left the house without at least one family member skimming it first, even the boxes marked boring. It slowed us down by about a day. It was worth every hour.
If you're coordinating a cleanout with family spread across time zones, ask the crew to hold anything that looks like documents, photos, or letters in a separate "review" area rather than sending it straight to donation or disposal. Most reputable companies are glad to do this — it costs them nothing and it means everything to you.