All is fair game
The thing is, particularly in our contemporary world, and particularly for an artist or writer who is consciously putting out works for public consumption (the "authorized" oeuvre (sp?) as it were), they MUST understand that scholars, biographers, art or literary historians, etc. ARE GOING TO FIND EVERYTHING if they can and use it in their books.
So, one would do well to bear that in mind as one goes about their working life as writer or artist. Which is not to say that one should run birthday cards or letters to lovers through the lens of self-censorship, BUT ... if one is "big" enough to merit publication, museum shows, etc., it only stands to reason that scholars are going to seek out the ephemera as well as the sanctioned output.
A lot of it must come down to honesty. If you have skeletons in your closet, if you're lying about who you are by putting out some dressed up image of yourself (Michael Jordan's a good example, albeit an artist of another stripe), then you're going to run into trouble over the long historical haul.
Sure, a lot of us write bad poems, draw bad pictures, create unrealized works of mediocre art, but we have to trust that, if we SUCCEED and actually produce EXCELLENT, VALUABLE work, then the examples of failure or mediocrity are simply that, failures and mediocrity, and what artist/writer/bussboy/line chef, etc. DOESN'T have that as part of their body of work?
Juvenalia is always accepted as juvenalia. I mean, Kerouac's teenage stuff is interesting but is, well, teenage stuff, and we can read it as such. We're not going to hold it up in comparison to "Visions of Cody", for example.
And further, I agree with the person (kerouacdylan?) who says they LIKE to read the diaries, the letters, etc. Look at Kerouac's collected letters. They weren't written with the public in mind, but reading them now, they're fascinating, as well as important in documenting his life.