i saved my bat quest for level 69...
simply coz you can get exp faster and easier from KQ (more % per spider KQ) at lower level...
and you get more exp from each orc in abyss at level 64, than it is at lvl 69..
(exp diminish from the same mobs, and you need more EXP for next level)
This is why I think saving quest DOES help... during each 10 level bracket, you fight pretty much the same mob... KQ and Abyss.
At level 64 say you get 600exp per mob, at lvl 69 you only get 550exp per mob, and you probably dont kill much faster. Hence u take longer to get the same amount of exp... That's my theory anyway
from memory... the normal level 69 quest + the 6mil quest leveled me to 70 instantly. (level 67 doesnt have any repeatables, and lvl 68 is the ugly lizardman mud quest, so u'll probably be sick of slow grinding by lvl 69)