My advice is to at least wait until the next patch. If I'm not mistaken, it's going to be a lot easier to get rep with those previously higher level factions (Argent Dawn, Timbermaw, etc) when it comes out. Even with that being the case, it's not the most rewarding thing. It really depends on how long you plan on staying below level 60 and without Burning Crusade.
Are you looking to just pass the time with a rep grind? If you are, I'd suggest finding something that can make you as much money as possible so that you have plenty when you finally do get the Burning Crusade. You'll need it for those flying mounts
Hope I helped a bit!