Question again m8s =)

[Blodrast]

yes, each province comes with a PT, as you call it ;)
basically, it will just give bonuses to the fellas you assign them to: loyalty, acumen, dread, and command stars.
In principle, you should assign PT's as soon as you get them, with a few exceptions:
- in the beginning, when you really don't have anybody but crappy worthless peasants to give them to; you can wait for a few turns until you get some hopefully better units.
- when you're waiting for a hero to pop up.

But of course, you can always assign them straight ahead, and them use a diplomat to remove them from the holders. Of course, you have to take into account that this will incur some loyalty hit to them, but most of the time it's okay.

It also depends a lot on the kind of PT you're assigning. Most of the time, you wanna give titles to units that have the most acumen, so that you can make more money.
Some of the time, you wanna give em to guys with a lot of dread - this goes especially for the more rebellious provinces in the game, such as Lithuania, Livonia, Portugal, Scotland. And for those few precious provinces that give you PT's with command stars, you probably wanna give them to generals who already have a few stars of their own, to make them even better.

Each faction has its own office (Chancellor, Earl, etc) which is a unique title, and can be assigned to a unit in addition to a regular PT.
There are other buildings in the game that give you titles: Admiralty, Constable's Palace, and Marshal's Palace. These can also go in addition to a regular PT on the same guy.

So, basically, most PT's you just give to whomever has a lot of acumen; they can be peasants just fine, you're probably just gonna keep them in garrisons anyway, which is even better 'cause they won't get killed in battle, losing you precious money with a loss of a high-acumen governor. The ones with command stars, however, you will probably want to give to good generals, front-line units, and most likely to cavalry (convenient from various reasons) - or heroes.

There is much more to it, but this should cover the basics, for now.
Feel free to ask around if you have other issues.