One other advantage to La Piazza instead of Guest Services is that typically at La Piazza they will hand over the whole book so you can see all of your options and then decide. At Guest Services, it is usually call and response, like, "What tasting do you want?" "Whiskey." "How's Wednesday night at 10PM?" "No, that doesn't work for me." "OK, then how about Thursday at 10:30?" And then if you want to book another the questions start over again. It is easier/preferable in my opinion, to just see everything they offer and then say, "OK, I want this one this one and this one."
However, the book is only at La Piazza during set hours. I forget precisely which ones. Other times of day or night, you need to go to Guest Services.
BTW, Guess Services is open 24 hours. We would often go ask questions (or get maps or sign up for things) at 10 or 11 PM. There was (usually) no line, and the CM would talk to you as long as you like and answer in depth because there was no one waiting behind you.