To answer your question directly, I would use JAWS with default settings in your target browser. If you can only afford one license, then use NVDA or Chromevox for your developers and give your Accessibility expert the copy of JAWS.
Keep in mind that while making sure your site works perfectly with a screenreader is very important, this only helps the blind. There are many other types of disabilities (e.g., hearing, motor, and cognitive disabilities) and to truly be accessible, your site needs to support those users too.
WCAG 2.0 is the best standard for making your site accessible to as many people as possible. There is A LOT of WCAG 2.0 documentation though, so I would start at webaim.org, http://webaim.org/standards/wcag/checklist if you are new to it, but do use the real thing http://www.w3.org/TR/WCAG20/ when you are ready.
Also, keep in mind that even if it "works" with a screenreader, it may be annoying (blind users rarely read top-to bottom, make sure you put in solid structure with headings and ARIA lankmarks) or it may not be giving a blind user the same amount of information that a sighted user might get. For example, helper text next to text inputs will be missed by a user tabbing through a form (fixes: hide a copy in the label with CSS, make the helper text the actual label, or use ARIA-describedby) - a good way to make sure it more than just "works" is to have your JAWS tester not be familiar with the site.