Is there a way to set an attribute to be hidden by default until the
user goes in and UNCHECKS the hide button?

My GUESS is that it would have to be done via an LDIF or something that
sets the attribute and the value for each user object in the tree.

But I'd rather not do that if I don't have to, as I'm also not sure how
we'd accomplish it on newly created users either (maybe have to tell
them that when the fill out the information to set it hidden)?