Can I use PHP header redirects above the Perch runtime inclusion?

I've added some code to my homepage to handle a redirect back to the homepage from a plug-in I'm using to handle a transaction flow:

<?php

  if (!empty($_GET['payment_ok'])) {
    if ($_GET['payment_ok'] !== null) {
      header("Location:reservation-processed.php?succeeded=1");
    }
  }
  if (!empty($_GET['payment_error'])) {
    if ($_GET['payment_error'] == 1) {
      header("Location:reservation-processed.php?failed=1");
    }
    else {
      header("Location:reservation-processed.php?unknown=1");
    }
  }

?>

Basically, I get the URL token appended to the homepage URL by the plug-in, and based on that token, I redirect to another page of my own, which has a PHP conditional to output different perch_content regions depending on which of my own URL tokens has been sent. I like this little system thingy, and it works great in my dev environment.

Published live, it was throwing these PHP warnings:

Warning: Cannot modify header information - headers already sent by (output started at /home/content/19/3871919/html/redesign/index.php:20) in /home/content/19/3871919/html/redesign/perch/core/lib/PerchUtil.class.php on line 1211

Warning: Cannot modify header information - headers already sent by (output started at /home/content/19/3871919/html/redesign/index.php:20) in /home/content/19/3871919/html/redesign/perch/core/lib/PerchUtil.class.php on line 1212

Warning: Cannot modify header information - headers already sent by (output started at /home/content/19/3871919/html/redesign/index.php:20) in /home/content/19/3871919/html/redesign/perch/core/lib/PerchUtil.class.php on line 1213

Warning: Cannot modify header information - headers already sent by (output started at /home/content/19/3871919/html/redesign/index.php:20) in /home/content/19/3871919/html/redesign/perch/core/lib/PerchUtil.class.php on line 1219

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/content/19/3871919/html/redesign/index.php:20) in /home/content/19/3871919/html/redesign/perch/addons/apps/perch_shop/lib/PerchShop_Session.class.php on line 15

Warning: Cannot modify header information - headers already sent by (output started at /home/content/19/3871919/html/redesign/index.php:20) in /home/content/19/3871919/html/redesign/perch/addons/apps/perch_shop/lib/PerchShop_Session.class.php on line 34

I disabled the warnings from being output. I tested and saw that it still works on the live website!

Am I doing something inadvisable, by messing with the PHP headers before Perch gets a chance to do its thing?

I notice Simon Clay and Jannis Borgers were doing PHP header redirects below Perch runtime inclusion a couple years back:

https://forum.grabaperch.com/forum/12-05-2014-redirect-parent-to-subpage