Need help with the movement+rotation C# script

I'm just starting to learn Unity and I've written a basic script for discrete movement + rotation in C#, using a movement script I found online as a basis. It uses WASD to move the rigid body around (without rotating it) and Q and E to rotate it 90 degrees left and right. It seems to work for the most part - however, the z coordinate behaves a bit weird when I follow a route that combines movement and rotation.

Namely, if I start moving from point A(x,y,z) and move forward->rotate right->move forward->rotate right->move forward->rotate right->move forward, coming back to point A again, the new z' coordinate is slightly different from the starting one. As a result, I arrive to point A'(x,y,z') instead of the original A(x,y,z). Same goes for rotating left.

That must have something to do with rotation, because if I only use the WASD movement (without rotation), my rigidbody returns to the same starting coordinates A(x,y,z) without any problem.

To illustrate, here are some screenshots. Starting position. The coordinates are (0, 10, 0).

Movement (small gif):

Resulting position. The coordinates are (0, 10, -1.490116e-08).

Where does the "-1.490116e-08" come from and how do I get rid of it? Here is my script:

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent (typeof (Rigidbody))]
 [RequireComponent (typeof (CapsuleCollider))]
 
 public class Movement_new : MonoBehaviour {
     
     public float speed = 1.0f;
     private Vector3 endpos;
     private bool moving = false;
     private float rotationVal = 0.0f;
     private Quaternion qTo;
 
     void Start () {
         endpos = transform.position;
         qTo = transform.rotation;
     }
     
     void Awake () {
         rigidbody.freezeRotation = true;
         rigidbody.useGravity = false;
     }
     
     void Update () {
         
         if (moving && (transform.position == endpos) && (transform.rotation == qTo))
             moving = false;
         
         if(!moving && Input.GetKeyDown (KeyCode.W)){
             moving = true;
             endpos = transform.position + transform.forward;
         }
         
         if(!moving && Input.GetKeyDown (KeyCode.S)){
             moving = true;
             endpos = transform.position - transform.forward;
         }
         
         if(!moving && Input.GetKeyDown (KeyCode.A)){
             moving = true;
             endpos = transform.position - transform.right;
         }
         
         if(!moving && Input.GetKeyDown (KeyCode.D)){
             moving = true;
             endpos = transform.position + transform.right;
         }
         
         if (!moving && Input.GetKeyDown (KeyCode.E)) {
                         moving = true;
                         rotationVal += 90.0f;
                         qTo = Quaternion.Euler (0.0f, rotationVal, 0.0f);
                 }
         if (!moving && Input.GetKeyDown (KeyCode.Q)) {
                         moving = true;
                         rotationVal -= 90.0f;
                         qTo = Quaternion.Euler (0.0f, rotationVal, 0.0f);
                 }
 
         transform.rotation = Quaternion.RotateTowards (transform.rotation, qTo, 100 * Time.deltaTime);
         transform.position = Vector3.MoveTowards(transform.position, endpos, Time.deltaTime * speed);
 
         }
 
 }

I'm new to Unity and programming alike, so I must be doing something wrong. Any comments/advice would be much appreciated.