Once a quest ends up six levels lower than your current level and if you are only interested in increasing your XP points, then it is pointless doing these quests and on completion gaining maybe just 1 point.
Main quests have to be done so once one is greyed out it may a well be left till later, concentrating rather on
higher level quests. I started "The Sunstone" quest and got full XP points but the quests that followed were six levels lower than my current level so I am starting the HoS quests and leaving "Battle Preparations" till later.

If you're only playing the game in order to get XP points, then... why? The game is a role playing game. If you find yourself in a situation with quests way below your level, do them first to get them out of the way. Simple.

Basically I had problems with Blood and Wine DLC in NG+ so I thought I would try to build up my levels prior to going to Toussaint.