I think it depends a lot on the teacher's individual style. I assume the book your using is fairly traditional in focus, so the dialogues and readings will focus on a particular grammatical point. Preteaching it to some extent makes sense because it will enable the students to then see it in use. Whether you preteach it fully or raise their awareness of the concept and how the grammatical form has a use is up to the teacher. I think that the grammar focus shouldn't just focus on the form used, but should also make students aware of the reasons for it and create the need to use it.