The way I see it, tasks are just bonus missions that you can get some rewards for, but they really aren't something you should include in your mandatory "to do" list. The way I play similar games is this: I clear everything that has any marks on the maps; those tasks that do not have any markings and are more like DAO Chantry Board quests ("Collect 18 of X and turn them in for a reward of a few gold coins"), I do not pay any attention to and only turn them in if I happen to randomly collect the required items while doing everything else.
I am a completionist, but I don't really count "collect N of X randomly somewhere" as a part of a completionist playthrough, as there is no clear route to completing them and it is a matter of luck more than anything else.