How things have usually gone with my games is:
Players A and B meet.
"Hi, let's kill indies and then whatever" says one.
"ok!" says the other.
*indies die*
*Player A attacks player B or C*
*Player B turtles or intervenes in the A-C conflict or attacks player D*
So I guess what I'm trying to say is you can rush someone the moment you meet them, but most people that I've seen prefer to kill indies first (more or less). Beyond that it's up to you. And yeah, diplomacy isn't binding. What is said and what is done have no need to be the same. That said most people seem to avoid outright lying, for what it's worth. I know I try to be vague enough to maintain a moral high-ish ground regardless of what I actually end up doing.
As for early aggression, people generally consider attacks to be of hostile intent regardless of province defenders (or lack of). Once a province has been flagged its under the sovereign rule of its holding nation. There are exceptions though. If you both attack a neutral province and you beat the other guy there, well, even if you did predict he'd go there and you'd kill him you didn't strictly speaking attack his province. Likewise I have seen and done some "corrective" expansion, which is when you attack another nation's province because it makes your lands more defendable (chokepoint etc.) or other reasons. Whether that's a declaration of war or not depends on how the other guy takes it and how you spin it. "I needed that farm because I have low income" is a valid reason to attack someone without warring intent. "I don't care, prepare to die" is a valid response.
Pretty much the only "sacred" thing tends to be people's cap circle, ie. the provinces immediately surrounding their capital. Taking that is not kosher. I mean you can, but pretty much everyone will consider that a declaration of war.
edit:
in addition to "lets kill indies" thing, it's not uncommon to also agree on a "fair" expansion while you're talking. "I take provinces 1, 2 and 3, you can take 4 and 5, is this satisfactory?". Of course if you are confident that your expansion parties beat the other guy's expansion parties, you might be intentionally vague or silent on the matter since you can just beat the other guy if you happen to take the same indies. Alternatively you might feel that you can take out some indies faster than the other guy. But again, you can be a bully or you can try to appear more agreeable and dependable. There's no set rule for how people act, I've seen all sorts.