This varies to some extent by class.  For science, math or other classes that depend heavily on the textbook, it is important to have the specific version used by the teacher.  For literature, we recommend using the specified ISBN, since a student may have some difficulty following page number references in class.  This is not required, however, and in many classes electronic versions or used versions from other sources may be a cost-effective option.  For any literature or other source material originally written in some other language than modern English, however, it is essential that the version used is based on the same translation/translator.