As you can imagine, most mortgage loan officers who don't work for USDA are primarily familiar with the Guaranteed version of the program, because it's all we can do. A 620 (or even 640 here in California) is a common minimum score requirement for USDA financing. The reason 640 is a common threshold is because that is a cutoff where easier underwriting guidelines apply pertaining to one's credit. USDA specifically states in regards to "Credit Worthiness/Derogatory Credit: Credit Waivers":
"Manually underwritten loans, and GUS loans that receive a â€œReferâ€ or â€œRefer
with Cautionâ€ underwriting recommendation, approved by the lender for
applicants with eligible credit scores of 640 and above do not require the
lender to submit supporting documentation to Rural Development."
And so it alleviates a lot of the underwriter's concern that the loan would have a high probability of being "guaranteed" by USDA, because they had reassurance that with a 640 score they don't need to be as tough (more or less meaning, "asking for lot of documentation") on underwriting. It used to be a 620, which is why a lot of USDA lenders are also using that as their minimum score. But not many go below 620. I still recommend you aim for a 640 score, just so you have an easier overall time in underwriting. At a score below 600 it probably wouldn't take long to get it up there, a few months, if there is a credit card(s) with balances then paying those off would likely do it, or a couple more months of on time payments reporting. Credit score isn't everything though, read the "Update" link below to read through the entire guidelines. It's good to know what they are, and not just take a loan officer's word for it (but make sure to understand that lenders add "overlay" guidelines, so not all USDA mortgage lenders are the same).
Another very important item to note, is that with the Guaranteed version, your monthly payment will be higher (even if interest rates do not move one millimeter) come October 1st (meaning your loan needs to pretty much be done by October 1st, like right about to close on the home), the reason being is that USDA is imposing a monthly fee similar to "mortgage insurance" that FHA loans have. You can read about the details, and how it would affect a USDA GRH (Guaranteed Rural Housing) monthly payment in a blog entry I wrote at: http://www.trulia.com/blog/shanethemortgageman/2011/06/usda_
USDA Guaranteed's main guidelines: http://www.rurdev.usda.gov/regs/regs/pdf/1980d.pdf
Update to guidelines in October of 2010, includes documentation requirements pertaining to pretty much everything, including the 640 score information: http://www.rurdev.usda.gov/SupportDocuments/an4543.pdf
These are "updates" (called Administrative Notices, AN's) to the USDA program that are announced, for example clarification of guidelines, general information, reminders, etc.: http://www.rurdev.usda.gov/rd-an_list.html
Best of luck!!
"you state you have perfect rental history and almost 12 months of car payment" this paints a picture of what might span 36 months or so...which is 1/10 the amount of time you will have a home. Because of this your credit score becomes valueable because it spans 10+ years, or at least 5+years, and if it is Low, it may very well be because you have not had it long. you need to work on improving your credit that will be your best solution, which should only take 6-12 months...