Yes, and no. If you send a message to someone you've most recently conversed with on iMessage, it will be blue. But if iMessage can't deliver the message, it will fall back to using text messages. I believe on the next attempt, the button will be green, but I don't have a way to test that right now.

No, it will ask you if you want to send as text. It doesn’t do it without you explicitly allowing it.

As recently as last weekend, I had it go through as green instead of blue without asking because the recipient was in a no-data area. Perhaps because I'd previously approved green messages for that person.