Apparently not if you haven't even hit 90 by the end of Dread Wastes.
I've left Jade Forest, V4TW, Kun-Lai, Krasarang as soon as I dinged. 85-86, 86-87, 87-88. Then I went to Townlong, finished that and started Dread Wastes. There are over 100 quests left undone approximately.