After reading this thread, I decided to save up all my troll frags. All I did was hop between kalimdor and eastern kingdoms collecting frags. I solved fossil, NE, and dwarf. I think the mount and pet hatchling for fossil were pretty low, got them really early. For NE, I got the robe and mirror early on too. From 250-450 or so, everything I solved was commons for mainly NE and fossil. At 440, I had about 600 or so troll frags saved up and I decided to solve 2 troll just to get to 450, and lo and behold the first assignment I get at 450 is zinrokh.
Eastern Kingdom is a great place for troll frags. You can use the zepp to go from the upper are to lower area (if you're horde) to gather a bit faster.
Now if only tolvir frags were easier to come by.
good luck guys merry xmas!