At the end of the day it's going to depend on your situation and what your business advisor/cpa/lawyer advises you of.
If you own a shop with custy's coming and going, employees or contractors working for you, there will be liability etc...
If you are doing it on weekends for friends, family, word of mouth (via friends and some new customers) requirements will vary.
If your mobile, you have a van, equipment, theft considerations, liability as you'll be on property other than your own.
Most folks on this forum will give different answers because every situation is different.
The best answer is to evaluate how your business operates, consult with your business advisor/cpa/lawyer and they should be able to assess your total risk/liability vs. situation.
I agree with Matt, but "taken to the cleaners" is relative term. If you burn through paint on a 3-9 year old vehicle fender, you're on the hook for fixing that.
To me, "taken to the cleaners" is a loss of home, hundreds of thousands, if not millions of liability (i.e. what doctors face), etc... I don't see how that can happen while detailing a car, although my attorney did site a plausible case where, you dress the pedals with something slippery/oily, (which I will never do) and as they drive away in their car, they hit someone or something because their foot slips off the pedal. That doesn't happen, the pedals are cleaned with APC and no dressings.