As we know from 'ce' prepended to equations in url slug?, any mhchem that gets edited into question titles affects the URL slug, e.g., http://chemistry.stackexchange.com/questions/5938/how-many-molecules-of-ceo-2-are-needed-to-oxidise-25-mathrmg-of-ce
I have been letting edits slide that make changes to the titles (originally we were going to try to use Unicode subscripts and superscripts -- see my "about me" section -- but that hasn't caught on).
Should we be more judicious about editing mhchem into titles, or does the fact that the questions look better on the page trump having a searchable url slug?