Update: According to a range of sources, Trash gives usual exp all the way to 5999/6000, the big trash that usually gives 50/55 will give all the way to 11999/12000, after that for the exalted ilvl 391 stuff you’ll need to kill lots and lots of firelands bosses.
If you’re a completionist and hardcore raider you’ll probably want to go on trash runs every reset day, usually tuesday/wednesday before your guild raid to farm to the soft cap at 5999 and pip it slightly over to the next tier to get access to the waist. Then every tuesday/wednesday if you want a heads up on your other guildies you can continue farming the 12000 range on the big mobs and roll on epic recipes and boes.
If you don’t have or don’t have the time to be in a full raiding guild raiding firelands, fret not. The old reputation farming days from black temple and ice crown citadel is back again. Currently you can pug a trash farm group to go into firelands to farm BoE epics, recipes and reputation. The trash are generally pretty easy if people know what they are doing and you can farm until honored reputation.
Once you go into friendly rep at 3000 you can get access to ilvl 378 cloaks which are generally a pretty big upgrade, but rather expensive to reforge into. At an additional 6000 you get honored which means ilvl 378 belts or waists, upon which you’ll find it quite tough to trash farm any further rep. Up til this point the tiny ones usually give 15-17 rep depending on rep bonuses, and the big ones give 50 or 55 if you have the guild rep bonus.
Even if you’re not in it for the rep, the drop rate for the BoE and recipes are good enough that you’ll likely see one to two drops every 1000 reputation gain or so. You can either resell then or use the 378 equipment like the Khaz Gorath trinket. If you see recipes on the Auction House below 1000 gold, and it is a good recipe you might even consider flipping it or learning it.