What is the best way to get XP from challenges? I know 5 star'ing on the first time is great XP doing the eveningstar challenges.

My question is more about how to milk the most XP from challenges.

Is it best to do them increasing the star count each time? example: first run 1-star.... second run, 2-star, etc.

Should you start with the level 20 version.... go up to 5-stars at 20... then do the level 21 and 5-star that... then 22, etc?

Or is it best to just 5-star when entering the quest at level 30 and never going back?

Thanks in advanced for your replies!