Dancing, just like any other sport is very subjective. There are many highly trained instructors for every dance. Whether it is Waltz or West Coast Swing, Country 2 Step or Foxtrot you will find that certain aspects of the dance remain the same no matter where you learn to dance and that others will differ.
Every dancer and instructor hears, communicates and interprets information in a different manner. As a dancer we should all be continually working on updating and improving our own personal skills. In doing so, we should expose ourselves to as many instructors as we feel comfortable. You will find that there are differences in opinion upon just how and when your body should move and how a dance should be danced. My advice to you would be to focus on what the instructor is saying while in their class and to give what they are saying a chance. Just because you have always done something a certain way, does not mean that there isn't a better more efficient way to achieve the same results. If after giving what you learned a chance, and it still doesn't work for you, then feel free to continue to do what you were doing before going to the class.
I have found that in every class, workshop or private lesson that I have taken, there has been at least one very crucial point made by the instructor that has impacted my dancing in a positive way. I haven't always agreed with what they are saying, but I have always attempted to at least give it a chance. I have also realized that sometimes it isn't What the instructor says, but How they say it. You may have heard the same thing over and over and just never been able to grasp what you have heard and apply it to you dancing and then someone else says the exact same thing in a different way and all of a sudden it makes sense.
Dancing in competition is no different than learning to dance in that it is also very subjective. You take two dancers dancing the same routine side by side and some of the judges will prefer one of the couples over the other. They could be doing exactly the same thing, with the only difference being a head tilt or the way one of them holds their hand and personal preference is going to influence all of the judges. Their is no absolute right or wring way to dance. Everyone interprets music differently, so learn your basics, become proficient and allow yourself to dance to the music!