The only time I take collection quests is when one of my toons is falling behind on xp, for example, deaths or when my 4 shamans need to do shaman quests my pally falls behind. I will do collection quests with only that toon, along w/kill quests, until they are all caught up.

And BTW, once you get your flight paths, going from zone to zone to do just kill quests takes little time and is well worth it.