We have done several in the past.....I don't think it undermines your business and skills, but probably does just the opposite.  Teaching one seminar or class never instills enough skills to be proficient, but with the proper teaching, customers can learn proper care and basic skills for simple vase work.  Most will not be able to go out and design the type of arrangements that would diminish your business volume. In our experience,  those customers become friends, have a better appreciation for designers, and better customers for the shop.  If we are like the bucket shops and discount stores and send our loose cuts out with little or no education,  we are really no different and they will buy from the nearest and cheapest source.  If your customer is not gifted in design,  they will probably never take the information from one seminar/class or even a small series of classes, and use it to your detriment.  They may even be a good source of advertising for you and your store.  We usually charge a fee, but sometimes do a simple small class for groups and charities to teach more about our art and flower care.