Yes, you can skip finishing the assassin/Sigurd quest. As far as ranking points go, this is one of the lower rewards (1500), and you can miss several quests and still be able to get to second place.
Kiya lists the ranking quests about half way through
this topic, along with how many points each is worth.
Check your messages (click 'My Stuff' in the top menu bar, then 'My Messages').
