The DLC will be installed/downloaded with the game proper, so you won't have much of a choice in that regard. What it adds is a handful of new missions involving a clan known as The Scorchers, so when you reach that quest line you can either ignore it until you've completed the main campaign or just plough through it right and there and then -- it's content-expanding, not narrative-expanding.
Edit: Come to think of it, I'm not entirely sure if you access the DLC quest line before completing the main campaign. Somebody who's recently begun a new game may be able to confirm either way.