Rule of thumb I follow is if it's an employee of someone's business then I will give a tip if warranted. If it's the sole
proprietor (owner of business) then I do not give a tip. So same would go with an instructor. Most likely they are the boss/ owner so I would not tip.
If I was a sole proprietor instructor, I wouldn't expect it and I would set my rates accordingly. Any tips would be gravy. I myself wouldn't tip for taking a group class. Private lessons? I would consider it.
Anytime I'm having a service rendered I consider tipping.. I don't think tipping should be required or expected in any profession but that it should be up to the consumer to judge whether the service is worth extra $... A positive review is a must in my opinion, especially for small operations like a one on one instructor since they really don't advertise and we all look for each others experiences.