Is it acceptable to take photos of slides at a conferences for private use? Is it accepted/appropriate to take pictures of presentation slides at conferences for private use? 

I am not talking about putting those pictures out in the public afterwards, I know this is illegal without consent of the presenter.  

Should it matter, I am talking about math conferences.  And I am asking because I don't see anyone taking pictures.
I have seen it done many times, sometimes I would see people recording entire sessions on video. My guess is these are people coming from institutions that cannot afford to send many researchers abroad and the one person sent will pass the material to their colleagues in their home country.

Some conferences explicitly forbid recording presented material. See for example the American Association for Research in Otolaryngology (ARO): 


  ARO   does   not   permit   audio   or   photographic recording  of  any  research  data  presented  at  the meeting. 


But that rule was not strictly enforced at the meetings I attended. So the answer is: it depends on the venue.