You're essentially using the fork tube as another BF wrench for the "flat" side. I wouldn't think on or off the bike matters much. But, I've yet to encounter one that was on so tight as to bend fork tubes. That would be scary. Anyway you are using the fork tube as the opposing wrench. You should have one hand on the tube and the other on the wrench. That way, minimal force is applied to the fork tube. The wrenches are arranged like the end of a scissors, where you either pull them apart or force them closed.
My MC shop friend puts the hex side in the bench vise and uses a big pipe wrench on the other end. But, I don't like the marks the pipe jaws leave on the part which have to be hammered/filed smooth again.
Cheers,