you should really only use a cosigner if your credit is so bad you can't get a loan on your own. generally, taking out a loan with a cosigner doesn't get you the lowest rate, and since your credit score is decent, the rate you'd get without a cosigner will probably be the same as if you took the loan out on your own.
Where can you find what constitutes "good credit". I see people mentioning the 700's as being good. Is there a scale/chart somewhere? He didn't know what bureau the dealer went through, but was told his # was 823. Based on comments here, that appears to be good - any chance any of that would rub off on my credit report?