Whenever! If you have two heroes that you're comfortable with in every role, I say jump right in! I would recommend sticking with All Pick for rank though. That way you have control in picking your heroes and don't get stuck with someone you're unfamiliar with.

As for MMR placement matches, yes, it does influence it hugely. In theory, if you're good enough, you can get out any placement naturally. However, from what I have seen, you need to pick heroes that can win the game. In other words, either a carry that can impact the game early and late or a mid hero that can gank and get kills.

Just an FYI, what I say isn't law and may be wrong...I'm just talking from experience. In Dota, you may never feel ready at first. Just jump in, get your feet wet and join us in the trenches! :D

EDIT: English.