Always snap to grid
From Unreal Wiki, The Unreal Engine Documentation Site
Keypoint is an abstract class. It is the parent of all classes that cause something to happen or have some effect at a particular point in the world.
None (beyond those inherited from Actor (UT), of course).
Keypoints have Advanced -> bStatic = true by default, which makes it impossible to move them by any means during the game, for instance by attaching them to a Mover (UT) (moving them in UnrealEd works fine, of course).
Roughly, Keypoint child classes are blocking actors, sounds, interpolation path points, and special actors that are only useful in single-player maps.