I wouldn't worry about it. I know tons of commission artists, and 999 times out of 1000 the customers love the finished product, even if the artists themselves are unsure about how it came out. If someone comes to you, hands you money, and asks you to draw something for them, you can reasonably assume that they have faith in your abilities, and whatever you end up doing will please them. The demanding, bratty customers are rarer than you'd think.
If you still have concerns, though, you can always instate policies for 'fixing' artwork that the customer is unhappy with, and/or having them choose the style they want you to use from the get-go, so you have a better idea of what they expect.