Tipping should be mandatory. 10%-15% and if you want to dispute, you have to email and wait till they get to your request

However, if "tip baiting" means that you get your groceries faster, than by all means go ahead. But keep in mind these people know where you live so if they put 2+2 together, you might be in for a rude knock on your door one day, by someone who has very little to lose.