If by ignoring, you mean non catching anything, I'm often okay with that. If an exception makes it top-level, you'll get a std::terminate and be done.

The "canonical" example people use a lot is std::bad_alloc. Often, there is no point in catching it -- what cleanup or fallback work are you planning to do when you can't even allocate memory?

Of course, silently swallowing exceptions with an empty catch-block is terrible.