I scoured the docs, but it doesn't look like this is something configurable. Instead, I wrote this little editor script that does essentially the same thing. (Note- It will always round down, but this could be pretty easily fixed)

Place this script in a folder named "Editor", and hit CTRL-L or open Edit/AutoSnap to access the menu.

@script ExecuteInEditMode()

class autoSnap extends EditorWindow
{
    var prevPosition    : Vector3;
    var doSnap  : boolean = false;
    var snapValue: float = 1;
    @MenuItem("Edit/Auto Snap %_l")

    static function Init () 
    {
        var window : autoSnap = EditorWindow.GetWindow(autoSnap);
    }

    function OnGUI()
    {
        doSnap = EditorGUILayout.Toggle("Autosnap", doSnap);
        snapValue = EditorGUILayout.FloatField("Snap Value", snapValue);
    }

    function Update()
    {
        if(Selection.transforms.Length > 0 && !EditorApplication.isPlaying)
        if(doSnap && Selection.transforms[0].position != prevPosition)
            snap();
    }

    function snap()
    {
        try
        {
            for(i = 0; i < Selection.transforms.Length; i++)
            {               
                var t : Vector3 = Selection.transforms[i].transform.position;       
                t.x = round(t.x);
                t.y = round(t.y);
                t.z = round(t.z);

                Selection.transforms[i].transform.position = t;
            }
            prevPosition = Selection.transforms[0].position;
        }
        catch(e)
        {
            // Nothing to move.
            Debug.LogError("Nothing to move.  " + e);
        }
    }

    function round(input : float)
    {
        var snappedValue : float;

        snappedValue = snapValue * Mathf.Round((input / snapValue));

        return(snappedValue);
    }
}

Here is the C# version.

using UnityEngine;
using UnityEditor;

public class AutoSnap : EditorWindow
{
    private Vector3 prevPosition;
    private bool doSnap = true;
    private float snapValue = 1;

    [MenuItem( "Edit/Auto Snap %_l" )]

    static void Init()
    {
       var window = (AutoSnap)EditorWindow.GetWindow( typeof( AutoSnap ) );
       window.maxSize = new Vector2( 200, 100 );
    }

    public void OnGUI()
    {
       doSnap = EditorGUILayout.Toggle( "Auto Snap", doSnap );
       snapValue = EditorGUILayout.FloatField( "Snap Value", snapValue );
    }

    public void Update()
    {
       if ( doSnap
         && !EditorApplication.isPlaying
         && Selection.transforms.Length > 0
         && Selection.transforms[0].position != prevPosition )
       {
         Snap();
         prevPosition = Selection.transforms[0].position;
       }
    }

    private void Snap()
    {
       foreach ( var transform in Selection.transforms )
       {
         var t = transform.transform.position;
         t.x = Round( t.x );
         t.y = Round( t.y );
         t.z = Round( t.z );
         transform.transform.position = t;
       }
    }

    private float Round( float input )
    {
       return snapValue * Mathf.Round( ( input / snapValue ) );
    }
}

No, there is not a toggle for this. And I do not know the best way to add this.