Threading is best done for fine hair (usually on the face), so if you have coarse hair on your back it wouldnt be a good idea. If the hair is fine on your back, well its up to you if you want it done. To do it over such a large area would be quite time consuming (and personally I think it would hurt more) compared to that of waxing. If they are willing to do it for you then perhaps give it a try, but dont feel compelled to keep doing it if you dont like it.