It is very rare, in my experience, for martial arts instructors to have much knowledge on correct warm up and stretching techniques. They usually do it the way they were taught, or have seen in other dojo, and this is passed on.
Professional sports trainers and physiotherapists change their approach as the knowledge base improves. I often hear coaches saying things like "we don't do it that way any more" due to new discoveries and scientific progress.
I might be taking a risk here, but I would say that martial arts instructors are probably some of the worst in this area, as they are very tied into tradition. The warm up and stretching part of the class is just to get ready for the 'real stuff', whereas for many sports professionals, this IS the real stuff and they try to follow current thinking on best practice.
Not many other physical activities resist change from traditional approaches to modern practices more than martial arts.
I would recommend to anyone that they spend sometime in a sports centre or gym, learning correct technique and take this into the dojo with you.