I guess it depends on what you are trying to do. While I do all my stuff by hand, usually based on models, real cars or photos, I have had to trace a photo on a project where the proportions were very important as well time. I didn't look at it as cheating as much as a tool to help me get it done acurately and quickly. If you think about it, how is tracing any different from the people who photo chop stuff? I know of a professional automtive artist who traces things digitally. Granted, my opinion of him dropped a little as far as his abilities, but his stuff still looks good. If you are trying to sell pieces of art and say you didn't trace it when you did, that would be a different story. Just depends on what you are doing.
Mike