I don't think you need to have any hard+fast rules either way. I've talked to hundreds of people on flights (and not talked to hundreds as well).

I've met some fantastic people on flights - I think 4 or 5 of my Facebook friends I've been sat next to and discovered we had a lot in common / worked in the same industry etc, so I am very happy we ended up talking. But of course, there are other days where I'm absolutely shattered or preoccupied, so the divider goes up straight after the security briefing without a word.

You do generally meet much interesting people down the back, and as I haven't flown economy for years, I haven't met as many, which is a shame (sort of).