A couple of years ago I went for an "L" Clearance in the big DOE world, and it is so very true not to hide anything. I was totally honest when I filled out my paperwork, then I had to go an interview with some guy from the DOI (Dept Of Investigations), and he had my entire life on paper. When the Feds run your credit report, they find everything, he had stuff that was 15 years old that "EVERYBODY", including the creditors had forgot about. It was amazing the stuff he asked me about.
Onething in particular was credit cards, they are very instered in people who get credit cards, max them out and forget about them (which luckly was not my case). But they do understand the inherit credit problems of us "Nuclear Nomads", how the seasonal work can sometimes make you late on that payment until you get back to work.
The interview went pretty well, lots of questions and lots of answers (all tape recorded), and then he bid my goodbye with my promise to clean up some recent credit oversights. I cleared up what I could and sent him copies of receipts and a memo on my status on some others, 2 months later I got my "L" Clearance.
So as far as the wonderful DOE, as stated in the above post. Just be honest and dont hide anything, they already know, they just wanna know how honest you are about telling them about it!