Modern iron gall inks (e.g. R&K Salix and Scabiosa, Diamine Registrars) are not as problematic as some of the older, more corrosive formulas. Even so, they shouldn't be left in pens for extended periods of time (several weeks or months) without flushing. Also, pens should be thoroughly cleaned before filling with different, non-IG inks. Of course, that's good advice for any ink.