Sorry but I'm calling BS on this. I am not talking mixing powders, bullets or primers. Cases are manufactured to SAAMI specs and reloading manuals have loads listed as starting loads and maximum loads. I realize that cases capacity varies from manufacturer to manufacturer but a max load is a max load regardless of case manufacturer. But that is also why I stated only if you are loading below max recommended charge. I see no reason why this is considered an irresponsible post. I've been loading for almost 50 years and I have never had an incident using mixed case brands for a specific load below a manual's listed maximum.
Now for the utmost in accuracy yes, mixing case brands will produce less accuracy due to the minor pressure differences.