For example, one could be an artist or a writer, but it wouldn't mean that one of these would be one's means of earning an income. As well, I wouldn't refer to writers or artists as being part of a social class, but I would still apply the expression "walks of life" in the case of artists or writers. I think it's possible to take the expression "walks of life" a little further than the the limitations placed on its definition by the American Heritage Dictionary.
Heinle's Newbury House Dictionary of American English This dictionary refers to "walks of life" as "positions in society", which I would say allows for more flexibility when considering the phrase "walks of life".
I would look in more than one dictionary to study and learn expressions. The MacMillan Dictionary lists "backgrounds" before "jobs", for example, in its definition: http://www.macmillandictionary.com/dictionary/american/walk_29#from-all-walks-of-life.
Here are some links to dictionaries:
American Heritage Dictionary
Cambridge ESL Dictionary
Longman ESL Dictionary
Longman ESL Dictionary Pictures
Heinle's ESL Dictionary\
Macmillan ESL Dictionary
You could do a search for Merriam-Webster's online dictionary as well. I don't really care very much for the format of this online dictionary, however, which is why I haven't included it here.