What is a good threshold for a UI which allows users to place objects, snapping them to a grid? 25% of the grid size? Less or more? Should the user be able to change this?
Yes, 100% with @AndroidHustle on this one. Photoshop is probably the most prolific example using it today; it also happens to be a feature I love dearly. A good snap threshold for me is around 10-15% of the grid size, any larger and it just becomes too much of a pain. Also, be sure to implement arrow-key movement as well (I use those all the time).