I'm choosing a charm for a Traveler's Notebook and am wary of flat metal edges rubbing the same spot on the cover. For people who carry theirs around, have rounded or softer charms been gentler, or does placement matter more? I'd rather not turn a bit of decoration into a tiny sanding tool.
I'd go for fabric or cord. Even a rounded metal charm can swing against the cover, so keep any hard bit hanging clear of the leather.
If you're open to an alternative to a charm, an embroidered patch could add a personal touch without anything rubbing against the cover—though it would be a more permanent choice.