Depends on where you live, but normally the answer is generally yes. Do you have a business license? Do you issue invoices now? Check with your city and state codes to be certain you are following the letter of the law.

Also, remember that once you file you go on the radar- so consdier it carefully. It is worth it to create a DBA (doing business as). That way you can easily have proof that your company (Piano Teaching Person Co.) is closed because there is appropriate paperwork that is filed when a business is closed. There isn't anything that you file when you just decide to stop teaching.

Ali

Most governments (state and local) are VERY interested in making sure that they collect their fair share of taxes and have offices or workshops to help you understand the law, and these are almost always free.

Whether your particular services are taxable is a matter of geography, and in some jurisdictions the same activity is taxed differently depending on when and how it occurs. In Washington state, for example, if I sell someone a book in one county the tax rate may be different than in another county, but if I offer a class which includes the book, the gross cost of the class is taxed at a different (lower rate) than the book would be. Goods and services are usually taxed very differently.

I'd strongly suggest seeking advice from your local taxing agencies and then confirming that with your (state tested and licensed) accountant. The truly paranoid will document these conversations for later reference.

Assume it is not a matter of IF you will be audited but of WHEN.

I'm guessing you're referring to sales tax. In CA, services are generally not taxable regarding sales taxes.
Check with your state Franchise Tax Board.