I have found it depends largely on the problem you are trying to fix. My experience has been that TAC will still do their best to troubleshoot issues with ISE when reservations are missing, but the moment a support bundle is submitted it will almost always get called out. If it is a performance related issue it may be one of the action items to correct. I've only had one case put on hold due to resource reservations, but to be fair, it was very clear that resource contention was an issue.
This line on the page you linked is important. It will fall on the customer to prove that there is no contention for resources while troubleshooting.
"If you choose to deploy Cisco ISE manually without the recommended reservations, you must assume the responsibility to closely monitor your appliance’s resource utilization and increase resources, as needed, to ensure proper health and functioning of the Cisco ISE deployment."
