I think the best way to look at ethics is "How would I feel if I were treated like this? Whether I knew about it or not." Gut instinct and comfort zone can really carry you a long way. If it feels wrong, it probably is...
As far as unethical requests, I'm a bookkeeper and had an ex-client ask me to 'blend' some of their personal expenses into their business so they could take more of a tax write-off. Which is the reason they are an ex-client. I won't do anything 'illegal, immoral or painful.'