I don't beleive we've ever seen her actually use the katana in combat,

though I may be wrong.  But the general assumption is that she's VERY
good with it.
	Actually, when Nodoka first pulls out her katana from
the 'case' she keeps it in, it slips out of her hand and almost
hits a terrified Ranma-chan. who happned to be BEHIND her at the
moment. :) Tgough we'd like to think Nodoka could do a neat
and quick seppuku, she porbably doesnt know the first thing
about using katanas.