Appropriate use of ligatures

Dear typophiles,

do you have some ideas on the appropriate use of ligatures like æ and œ? If I decide to use them in cases like "mediæval" or "œuvre", how much consistent should I be in forcing this spelling in quotations, in titles of various works (for example, if an article is cited which is titled "Sets Are Haecceities" (thus spelled), should I retain the digraph or should I force the ligature on it? Likewise, when transcribing Latin, should I keep the original spelling in the sources (and their titles), or should I unify it to my style?

Thanks for your ideas,