I feel the biggest factors to consider are whether you want to use Imoen or Sarevok. If you intend for one or the other to be in your party, then completing the Watcher's Keep before you recruit them means they will lag behind the other party members by a very significant margin. This goes double if you intend to dual-class Sarevok (the
only way to get a Fighter/Mage or Fighter/Thief NPC) since he just won't have enough XP to catch up to the party without the adventuring detour of the Watcher's Keep.
Unfortunately, waiting to the start of ToB can be rather late
Hickory: [Mellisan]
* "But Gromnir is mad. He has barricaded himself in the castle throne room, and his troops run amok through the town with no regard for the lives and rights of the citizens of Saradush."
[Player]
* "Such a betrayal is worthy of death!"
* "I should break down the castle walls and slay this Gromnir for his cowardice!"
* "I like this Gromnir's style!"
I mean... please! *puke!*
Shoe-horned conversation paths are sadly quite common throughout the genre.
Baldur's Gate 2 avoided a lot of this by forcing conflict. Irenicus was out to get you, and was an enemy whether you liked it or not, so the only motivation it needed to force on you was the "get into spellhold" part. Few other games manage to string themselves together so flawlessly.