I only accept Merchant's Guilds if I am planning on sending a merchant force down to Timbuktu or Dongola, or sometimes even the gold mines near Zagreb.

I always accept Swordsmith's Guilds since they are extremely useful if you want to train an army. I don't usually bother with Assassin's Guilds, unless I am a Muslim faction. Theologians Guilds are only useful if you need to boost your religion in a province (through priests), or maybe in a city with a good Cathederal since it means I can basically train Cardinals.

I very rarely get Horsebreeder's Guilds. And having played as nearly every team, I've only ever once got a Mason's Guild. It was in Yerevan, can't remember who as though. It doesn't actually make all buildings cheaper, only walls and towers, though it does give you a big discount on them.

Explorer's Guilds seem pretty uselss, I only accept them if I am stinking rich.