It does matter to me that attention to detail be given as much as is possible for films or programs at the cinema or on tv. Unless it is complete fantasy like the recent new Clash of the Titans with the Ozzie accents which does sound slightly funny and then they put Pete Postewaite who does the best he can, acting really well, as he does. It is entertaining to see errors and outtakes but there are enough special programs that feature the mistakes already.

With books it all depends on if the reader wants something factually informative or just light-hearted reading not to be taken too seriously. Good spelling in reading books should be important to show good example especially to young people who might pick it up to read and think that it is the correct way. If it is copying a dialect I don't have a problem with that.