The first time Idaho delegate Joe Williams attended a Democratic national convention, he cast his vote to help nominate Franklin Roosevelt for a fourth term in the White House. Attending his ninth national convention this week, Williams, 84, says he's every bit as thrilled. ``It's still just as exciting. We get all the hoopla and hurrahs, even though it's cut and dried like this one,'' said Williams, Idaho's auditor for 30 years. Williams said the vote he plans to cast for Michael Dukakis will be his last as a national delegate. He said he plans to retire from politics when his present term expires in two years.