Actually, I believe there's at least one comic from the 70s that shows him with the "perma-grin." It's one of issues from The Joker series that features a conflict with The Creeper. I know it's reprinted in the earlier editions of The Greatest Joker Stories Ever Told and Stacked Deck. Given how much of a fan you are, Klaus, you can confirm as I'm certain you have at least one of those books.
Personally, I've never liked the "perma-grin" approach and agrees that The Joker is portrayed best as just smiling a lot.