Skip collection quests if you like. Just understand that in Northrend, you're hurting yourself since there are many long quest chains that involve many collection quests. With the nerf to the leveling curve in Outland, you cna do what you feel like. Northrend is just not so, unless you're willing to spend time in instances quite a bit because of the number of collection quests in each quest chain. I'm doing them right now and there is just no avoiding them.