If it's a quiet shift and they're a nice customer, then asking for a little sip of a craft beer is totally acceptable. Trying new things is fun and shouldn't necessarily cost you money. You get it. But if the bar is three-deep and they are asking for samples of everything on tap, including, like, Heineken, which you refuse to believe they've never tasted before? Totally unacceptable. But you'll do it anyway because it's hard to stop giving out samples once you've set a precedent.