Blocks spawn only on grid

I have a game where the blocks fall from the sky to the players last position. But I want the blocks only to fall on a grid based system that way you don’t just have a pile f blocks. Here is my current script:

public class gameMechanics : MonoBehaviour {
    public float cubeSpawnTime;
    private float cubeSpawnTimeNorm;
    public Transform player;
    public Transform fallingObject;
    public float fallingObjectHeight;

	// Use this for initialization
	void Start () {

	// Update is called once per frame
	void Update () {
    if(cubeSpawnTimeNorm <= cubeSpawnTime)
        Instantiate(fallingObject, new Vector3(player.position.x, fallingObjectHeight, player.position.z), Quaternion.identity);
        cubeSpawnTimeNorm = 0;

Assuming you have a unit grid, all you have to do is round the values:

Vector3 pos = new Vector3(player.position.x, fallingObjectHeight, player.position.z);
pos.x = Mathf.Round(pos.x);
Instantiate(fallingObject, pos, Quaternion.identity);