Common sense and common courtesy.
Neither is a reportable offence. However, sometimes a pick by either of those is a clear indication of harassment or trolling - for example, someone calls mid first, but a higher pick takes it. The first caller picks a mid also.
Usually that kind of behavior will also continue in the chat, and that's what will determine your judgment. Champion select is not something you have much information about in a Tribunal case, so it's not a solid reason by itself to Punish. However, combined with other information, it can complete a picture of a player as toxic.