Another thing you might want to consider is the use of rel="canonical". If you use the canonical tag on each job page to point back to the appropriate category it will help those category pages rank better rather than spreading the juice out among the individual job postings.
Matt Cutts recently did a video about this practice. He talks about product pages, but it should be the same in principle. You can find the video here: Canonical all product review pages as a single url.
You will still want to handle the missing pages with a 301 or 404, but there will be less concern about losing juice every time a job is filled. And as the video says, this is something to consider but it isn't a solution for everybody.
