Contrary to most of the loud people in here, I rarely get assholes or trolls in my games. Normal, Ranked, Bots, or otherwise. I would say ARAM is where the most abrasive people I run into are ironically, but even there it's 1 or 2 people per 10 games. Every step Riot takes towards curbing the assholery does have an impact, but there's so many out there it takes a while. There's also different things that offend different people but I barely use ignore.
Now, when I duo queue with Ronne, I've noticed the player attitude drops significantly, so it could very well be rating based if our ratings are that different (our ranks are the same so I don't know why that would be). It's cliche but the best way to prevent assholes is just to be nice and friendly from champ select on. It's not foolproof but it's better than just being quiet (or egging them on)