I have a level 3 account and posted my links many times, however, I actually invest time in reading the question and answering it properly. If the question/answer is relevant to your site and if you really helped that person out, there is no problem in leaving a link as a reference. However, I've seen many people just post something like "Glad you are using yahoo answers, keep up the good work", and then they post a link in the source section. Well, in such cases I don't understand how some people are surprised by the commonly occuring bans. Answers like that are ASKING for a ban
Yahoo Answers is a great place to find topics for blog posts, and it's also a great place to promote your blog but you should pay attention to what you post because you could get yourself banned if you abuse.
I think you can post the url in source and only put url of your site in body if and only if you have relevant information on your blog or site according to that question....