When my guys are able to advance in the guild, should I have them do it? In other words, should I elevate my warriors to Samauri, Paladins, etc, as soon as possible? Or should I wait until I finish all the Warrior Guild quests in Valeia first? Same goes for the other races: should I finish all the Priest quests before I change him to a Monk, or whatever?

Or can I embark on the Guild Advancement quests at the same time as embarking on Guild Acension quests? Should I basically just accept every single possible quest and work on them all at the same time? Please help! Thanks!

One last thing: I'm not too far into the game, but I have a rogue who isn't allowed into any of the guilds in Valeia. Does she just need to build up some attributes, or are rogues not allowed to join any of the guilds there? And if not, how the heck will she ever advance to being a Ninja or Ranger or whatever? Thanks again...

Basically I would ascend as fast as possible, as the elite classes are much better. The possible exceptions to it are Priest and Wizard. If You don't have a ranger (and don't plan to use one), The Priest is the only one able to use Vine spells. If You change to monk after the priest reached level 2 (as did I), Your monk's vine spells are limited to Level 2 - all the good ones, as cure poison are not available to you. It's the same with the wizard and the warlock with Stone spells.
And yes, I would take every advancement quest possible. I would only take the ascension quests if You really plan to change classes or to gain access to specific guild. A thief can make bard and so gain entry into the wizard's guild. Of course, if You don't really want to change to the bard, You have to cancel the quest after the entry to the wizard's guild.
The thief has it's own guild, the pawn shop. Only there isn't one available at Valeia. You have to reach the second town for it.

I have noticed that after I advanced some of my characters (paladin, ranger, warlock), they no longer receive attribute points when they go up a level - only one measly skill point. So, I wondered what was up with that. I am raising attributes on my warrior who will become a ninja big time before seeking advancement. As for the rogue . . . did you go to the ruins SE of Valeia? Talk to Gorthius? Or did you meet him and the three jade thieves and off him right away like I did the first time? Seek the Ish town for rogue guild.