Maybe he was referring to not mixing AGM and wet cell batteries...not starting battery vs deep cycle like you're thinking. Most of us use a "cranking" battery for starting and then one or more deep cycle wet cell or AGM batteries for stereo/lighting.
I'm pretty sure you don't want different size batteries on the same bank if you are using a charger though (like having a group 27 and a group 31 on the same bank powering your stereo).