Be selective about the Quests you do. Kill Quests are great for XP. Kill everything in between points A & B, time you spend running is time wasted, If you are gaining experience whilst running, its more efficient. Personally I don't think that the rule of "not doing ANY collection quests" is valid, you have to weigh up each quest on its merits.

For instance: You get 100% of the quest experience if the quest is (upto) 5 levels lower than your characters level. You also get experience for grinding critters' to get the items. As long as both these criteria are being fulfilled, why shouldn't you do these Quests? It'll save having to do rep grinds at a later date. The only obvious expceptions are the Quests that involve having to pickup loads of ground-spawning items.