I don't think it matters. Friend of mine joined the marines without them bothering to check his credit record. Or they did and found nothing unusual. While I have no idea how bad his credit record was, he dropped in and out of college twice, had no steady job, and love partying and his parents ain't rich, so I don't imagine it was shiny. Of course, that was ten years ago and it was the marines, so policy may be different, but I don't think so.
I believe they do check your credit record before they issue you a security clearance for sensitive stuff. But since when you enlist you will start at the bottom with no access to sensitive information, I don't think this matters. And by the time you climbed up the ranks enough for it to matter, you should have paid the debt and only have whatever's normal for someone of that paygrade.