Of course the first thing we want to do upon hitting level 100 is to start gearing up. And we want to run heroics, which means getting a silver in proving grounds. Well, while you're at it, don't miss this quest, given in your Garrison by Lieutenant Thorn if you're Alliance, and Warmaster Zog if you're Horde. It's a good way to get your hands on a solid 610 weapon (almost as good as an heirloom at level 100) and, if you get lucky and get an epic instead of a blue, it's actually ilevel 630 and better than the heirloom weapons from Garrosh.

I did Bronze as DPS first, and found it to be pretty easy. It's basically a free weapon as good as a dungeon drop, with a chance to be as good as a heroic dungeon drop. Since you're going to want to be able to queue for heroics anyway, there's no reason not to get it done.