John Bogle has suggested everyone should own a percentage in bonds - if only to ride out the bad years in stocks. In theory people can tolerate 100% stocks, but in practice they should have some "ballast" in bonds. So what does that mean to someone with $10,000 going on $15,000?
I'd suggest 20% bonds while you learn / experience what you do when stocks take you for a very unpleasant journey. I believe Roth accounts have a $1,000 minimum investment, so you could already buy into the total bond market fund. But if that's not the case, at $15,000 in assets you can set aside $3,000 (the normal fund minimum) for the total bond market.
There's a lot of MMM people who like 100% stocks, but at this point in a bull market its hard to tell who's talking and who means it. That's why I think a 20% bond starting point makes more sense - it fosters better behavior if the market corrects, and you learn that stocks and bonds can do better than each other at different times.