You can't tk before the round ends period, but I know that when I'm in-game, I show some leniency depending on how many people are left, how disruptive it is to the team's winning %, joking vs malicious/complaints, etc. Sometimes, those players may just not be in a good mood or may just be griefing to them.
Zeeno, you haven't been banned for tking in a year, but you can get banned for tking an unwilling teammate before the round ends (and even willing/joking around right before it ends, depending on the situation). Will leave this as a warning.