I usually do collection quests if they:
1 - provide a gear upgrade
2 - lead to another quest that provides a gear upgrade
3 - provide a rep increase of a faction I want (which usually means access to better gear / enchants)

IMO - upgrading gear is one of the most important parts of levelling, since it effectively allows you to power through kill quests faster and makes instances feasible / faster. This is especially true when moving into Outland or Northrend for the first time, where the gear jump is substantial.

Cheers,
S.