I have my players write them down. When I ran Sailors, one of my players was Born on the Battlefield. He wanted to roll a knowledge check regarding the history of the keep. I allowed him to do so, as he likely heard stories of wars due to his birthright. Even if the modifier is not useful, incorporating that element into their backstory is pretty cool, I think.
The Judge is also completely in his rights to assign a +1 bonus to the birth auger later. This is especially appropriate if events in the campaign make use of the auger in some way. For example, if an adventure takes place within (or including) a storm of mythic proportions, a character with The Raging Storm as a birth auger (and who survives!) might discover that his bonus has been increased by 1.
Random rolls in this game drive the action, and our understanding of the action in the game. Does it matter what the manifestation of a spell is from a rules perspective? Probably not. Does it matter to experiencing the game? I think it does. And you never know...sometimes the manifestation might actually be important. So to with the +0 birth auger.
Imagine that a character has a +0 birth auger "Survived a spider bite". Couldn't that auger his later opportunity to win the patronage of Mellythese, demon-goddess of spiders?
What about our baker who was raised by wolves? Might he not be able to make a Luck check that he knows
the wolves now attacking his village? If so, they might not attack him personally
, so long as he doesn't attack them
. Would he be able to challenge the Alpha Male of this (or another) pack for dominance....as Tarzan does with many groups of apes in various ERB novels?
Let the player
propose how his birth auger applies. As a Judge, consider how birth augers might apply to (or be affected by) the adventures you run. And the player who didn't bother to roll, because he had a modifier of +0? That character is literally out of luck. Nothing special happened at his birth/in his youth.
(But he might want to roll next time!)