Sometimes I do run across an apple that doesn't quite sit flat when sitting on its stem. In that case, I'll usually do a little trim of the part of the apple that's sticking up and then cut the apple. It does work. It's so much easier to line the hole in the center of an apple slicer up with a flat apple than one that's sitting on its side.
Many of you already figured this trick out, but I thought I'd share anyway. After all, you never know who's going to be one of the lucky 10,000.
Obligatory xkcd comic:
(If you're one of the lucky 10,000 that's never read xkcd, then get ready for some fun . . . and make sure you have time to waste before you open that link. You're going to need it. )