Auto snap to grid in Unity
If you need to auto snap a gameObject to the grid in unity this script will help you.
Usage:
Create a Editor folder in your assets folder.
Add the AutoSnap.cs script to the Editor folder.
To open press Command + L (Mal), Control + L (PC)

Source: http://answers.unity3d.com/questions/148812/is-there-a-toggle-for-snap-to-grid.html