@bump I think anything that's obvious as crossposting shouldn't be crossposted. That mostly means referring to "this platform" but broadcasting to another platform, where the post doesn't make sense. Also tagging people, since those username mentions will look weird off-platform.
It's also bad form to post things that invite reply or seem social/conversational, but you're posting to unmonitored accounts.
If I can't tell you're crossposting, it seems harmless.