It is true that, in order to achieve a good success rate in the EFL/ESL classroom teachers should select a model variety of English to ensure that the type of EL that is being taught reunites the most universal and general characteristics as possible. For this reason, some educators prefer to use what they call the "network" English. This type of English is called "network" because its usage is as free of colloquialisms as possible, much like the newscasters use in their networks to tell the news.
The reality, however, is that there may or may not be an ample availability of teachers who possess enough control of the English language to make it that general and universal. Moreover, each student will undoubtedly adopt their own version of the target language, adding their personal traits and cultural characteristics to it, regardless of the model variant.
Current practices follow what is known as the "World Englishes Initiative", which follows Modiano's model of English citing that English should be treated as an international language, and its usage will depend on locality, culture, need, or community. Another common initiative is Kachru's model of Three Circles of English, which places English usage as a variant in inner to external societies.
This being said, the best choice is to expose students to a variety of accents and expressions in the target language. The English spoken in Ireland, Scotland, North America, and England can all aid in the processing of language differences. As we can see, language ultimately is a product of the speaker, and not of the teacher.