Good question, and I'd say it depends on your playing style. Personnally, I like to focus on the Offense, and rely more on dodging for defensive purpose, and it works, but I'm a Ranger: I usually fight with a bow and the bad guys often focus on my Fern Mastiff rather than me. A Guardian, who has no pets and is big on melee, you may need more Vitality/Toughness than a Ranger. But keep in mind you won't hit as hard as a Power/Precision build.
In short, figure out how much Offense/Defense ratio you want, and make your build around that ratio.
In story-telling, unless it is stated later on that a piece of information was actually false, every piece of given information is truthful.
With my guardian I went with power & toughness; and I split the 3rd stat between healing & vitality. Reason being that guardians will never benefit from vitality as much as a warrior would, and the healing helps with all the support they offer. But you still need at least some vitality or you'll only be able to take a few hits. Only problem with this is power / tough / vit gear can't be crafted, you have to either get it from dungeons, drops, or buy it.
Personally I wouldn't mess with pumping up precision because guardians aren't the best profession choice to play offensively, but if all your ever going to do is swing a sword I guess it'd be worth it. Precision doesn't make much difference unless you also boost crit damage % though.
With exception of my glass cannon Ranger, I've taken power for quicker kills and toughness for more defense (except Guardian, with which I took vitality because of the already high armor). As a strict PvEer, the power helps for quicker kills and the toughness keeps me on my feet.