When I first started fishing in Zangarmarsh around level 49, I was too low to turn in Strange Engine Parts, which has a minimum of level 55, so I just banked them. That's why at level 55 I turned in five in a row.
With patch 2.0.3 coming before WotLK, the XP needed for levels 61-70 is supposed to be reduced by 40%, so it will be better to save up engine parts (and any other XP quests I finish before then, such as "I'm Saved" if I'm at the right place at the right time) and not complete these quests until after the patch. For a comparison at level 64, one Strange Engine Part turn-in right now is about 1.8% of the way to level 65, but it will be 3% after the patch.
I also used the new Exploring Achievements on the PTR to find a few areas I missed the first time around, and when the patch goes live, I can get a few hundred more XP exploring the area behind Tyr's Hand, the new Death Knight starting area.