What time you play might also make a difference. I've encountered a bot 2-3 times after queueing for 50 minutes or so at 10 AM on a weekday. What happened was probably that matchmaking took me + 1-2 high MMR player in the enemy team and put us with a bunch of lower MMR people, including a bot. That's kind of annoying, but I reported the bot every game (2 of the games I'm pretty sure it was a bot, one of the games I'm only like 75% sure).
So: Get to level 30, queue at reasonable times, win a lot so you get out of bot MMR. Oh, and report bots whenever you see them. For some reason, the bots I've encountered have all been level 30. You'd think the bots would stop playing once they reach level 30.