It's not about how many quests are left...you'll have them no matter what, it's about not wasting the GOOD quests too early. Now if you wanna grind mobs at 58 that's fine, but why waste the hellfire quest exp that early when you're still benefitting from the 20-60 exp boost with so many easy quests left in azeroth? Finish what ya can then come back and cash in on the good exp a lil later so you're not too behind lvlwise.