well, i've mistakenly plugged other headphones in the line out multiple times over the years, and it never happened. It actually always sounded quieter than the HP out at the same volume level, for some reason.
If that's the only reason to be weary of it, i don't see a problem with going with the line out, i could even set a lower max volume on the x3ii to be even safer.
there's always a risk of short circuiting the headphones. as long as you don't mind the risk, by all means. but i personally would never recommend plugging anything other than line input level devices to a line out