Premades (teams that are arranged in advance, often using voice chat): if someone asks you if you are a premade team (they should ask if they have any concern), tell them the truth. If you don't, you are being a jerk.
PUGS (pick up games): be alert if you don't want to be in a premade. Ask them if they are premade. If they tell you they are, you play, lose, and then complain... you are being a jerk.
I've asked several times if the game I joined is a premade and everytime the host appears to tell the truth. Other times, if it seems crystal clear that the game I joined is a premade (without my asking), I just leave in the lobby. Sometimes, they tell me its a premade and I play anyway.
What's the worst that could happen? Your stats could get screwed in a system that the pubs alreadly admit is jacked up?
I think we can all agree that a premade team with 3 people who work well together SHOULD beat any team of 2 and a random. The competative matchup would logically be 3 good premades vs 3 good premades. I can easily say that a team like Shadow and Itchydustbin, et all are bound to win by playing against just anyone, but honestly, we all play this game because we enjoy it. If they only played when there was a premade team up to snuff, then they'd MAYBE play weekly (if you are interested in creating a premade to crush them, let me know). But that said, I don't fault them in the slighest for taking their premade on the road and kicking some butt. Do I think that people like Shadow, et all, could get 100 wins just playing customs without their teams? No. Not a chance in hell. They'd have to be insanely good AND incredibly lucky due to technical stats. Prove me wrong by playing 8 games without your team. But, you don't have to because I'd enjoy to see a 200 win streak.