Is what's on the I-129F a reasonable plain language description of what you do, even if it's not the official title?
For example, if your title is "Tour Specialist Grade II" and your fiance wrote "Tour Guide", no worries.
But if you are actually a tour guide and your fiance wrote that you are an "Investment Banker", you should prepare to have your relationship questioned, and provide not only proof of what your job title actually is, but also proof that your fiance knows. For example he could work it into his updated intent to marry letter by saying something about you taking a vacation from your position as (insert correct job title here) to spend time together when he met you person. There are lots of anecdotes of being asked about the American's job at the document check stage of the interview, which is to make sure (a) the American has been forthright and (b) you know him well. In your case, you could have to prove you've been honest with your fiance.