I did see someone once try to tip one of our instructors ... she of course refused the money.
Out of the 2 1/2 years I have been dancing with my instructor I have sent her flowers twice. Because of my job I get hold of some freebies sometimes and will bring those in to the studio for all the staff to enjoy. We have a few bakers who will cook a meal or bake some goodies and bring in for the staff sometimes. There are different ways to show your appreciation, just make sure it is done out of APPRECIATION not out of some sort of preasure to conform to "social protocol".