Bah, order what you want when you want and don't worry about it. There are no posted minimums and if the waitstaff scoffs they just get less of a tip and can deal with it. Hate to be so blunt (well, not really LOL), but there is no semblance of shame in it. I wouldn't get a PS at the California Grill and order nothing but a diet coke, but just appetizers or just desserts is acceptable anywhere.
Part of the fun of WDW is so many different eateries and they see this all the time - people will actually get apps one place, an entree somewhere else, and dessert in yet another place. In larger cities that's actually close to the norm, and WDW is no exception (it really is a big city if you think about it hehe).
That said, I wouldn't line up 3 PSs in a row, hehe, but in fact I *do* have a PS at Sci-Fi during our next trip that will be dessert only. I will inform the waitress when we are given menus - if she wants a nice tip she or he will smile and say "great!". I know I always tip more in instances like this - up to 25/30% for an app and some drinks.
As always at WDW eateries, there are CMs that get it (that the more magical they make our visit to their restaurant the bigger tip they get) and those that don't (those people who deserve little tip because they roll their eyes at dining plans, or give you a dirty look if you only order an ice cream sundae.) Reward those who do, and leave a minimum for those that don't.
(Hehe, was that soapboxish? Just don't think anyone should feel the need to miss out on some of the stellar WDW dining just because they weren't hungry enough!

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N.E.D., fighting for the right of dessert-only PS's everywhere
