I tend to do a little specialization. I'll train all my merchants at a major port city to get a merchants guild there. I'll train all my spies in one place to get a thieves guild there, and do the same with assassins if I'm not playing a chivalrous game. All priests will be trained in one city in each geographical region to get a theologians guild. Other than these, it really depends on what faction I'm playing. In my current game as England, I wanted 2 castles on the British Isles, one with a Woodsmen's Guild and one with a Swordsmiths' Guild. I did the same in Mainland Europe. Once I've got those, and taken one explorer's guild someplace, I tend to accept whatever is offered aside from Thieves' Guilds. In fact, almost ever city I take has a Thieves' Guild, which I wind up tearing down to replace with something else. Sometimes I'll take a merchants guild in a town, recruit a few dozen merchant cavalry (disbanding them after recruiting), then tear down the merchants guild in the hopes of getting a Horse Breeders Guild.