I don't use it in my own job, but Matias Brutti and Mike Ridpath of IOActive did a presentation here at B-Sides PDX this last weekend on how they use SE, along with recordings of calls they've done on the job:

http://www.ustream.tv/recorded/17736407

Jayson Street did his "Steal everything, kill everyone, cause total financial ruin" talk at DerbyCon two weekends ago about his use of SE during his night job:

http://www.youtube.com/watch?v=8esU0G5zlRU

There is also a really good book on SE called hacking the human.

its pretty new here link on amazon http://www.amazon.co.uk/Social-Engineering-Art-Human-Hacking/dp/0470639539/ref=sr_1_3?ie=UTF8&qid=1318500808&sr=8-3

if Social Engineering is defined as "an interaction with a person that causes them to give you information that may not be in that person's best interest but is used to benefit the person asking"  (and imho this is a good definition)  then technically, no, i don't use it. I don't interact with them until i return their call/e-mail.

That said, when people contact me to hire me, I have a whole series of things i do to investigate them before I accept or reject a job. For the most part i want to make sure that I'll be in a safe situation, but it's also good to find out whether they can afford me!   

Also, finding out as much as i can helps me to make sure I offer them what they're looking for.  I always say that "affordable is relative" because what one guy would pay $100 for, another guy might pay $1000.  The second guy is expecting a certain level of service, and think that $100 is too cheap. My approach is to accept more clients that would be fine with paying $1000, and make sure those folks felt like they got a good deal.

Kind of a roundabout way of answering your question but i hope that helps.