As a benchmark, some places I've been at over the years didn't offer extras, but charged for the 'tug', usually asking $20 to $30, which gives an idea what some of them expect tip wise. I guess the tip will vary depending on whether it's a place offering a variety of extras or not. At a place that offers a lot of extras, but you're not interested that day, or budget can't handle it, if you're paying the basic fee up front to the attendant, might be worth-while offering the tip up-front (if you know her and have been pleased with previous experiences); otherwise if you decline extras, they won't know a tip will necessarily be coming and you may end up with an uninspired, even unpleasant, yank on your chain.