I've heard that before - various incarnations of "rolling tubes in this slot won't affect the sound", because "they're cathode followers with 100% feedback" etc. Still haven't met a slot that doesn't affect the sound. It's directly in the audio path.
You can have components that are "voiced" for the specific tubes it ships with. Sometimes components have a strong voicing on their own. In these cases, tube rolling may not readily yield satisfying results - but it WILL affect the sound. Always. Now is it a useful endeavor; a good use of time & money? That's for you to decide.
It's not surprising some manufacturers would discourage rolling. When you owe warranty support on these heavy, complex amps - do you really want users rolling them with power tubes willy-nilly, from unknown sources and questionable matching? Plus they may sell their own branded (labeled) replacement tubes, at healthy markup.