From what I've read, I think that when you create a character from scratch it randomly chooses you "past decisions" for you. That or it makes an educated guess based on the traits you choose. A "ruthless" reputation for instance might be more prone to renegade choices. Though, to be honest I don't think they've full explained that side of things to date.
This contradicts what I've read - my understanding is that at certain points it will ask you what you would have done in certain situations.
I'm just going to carry on my Evil Bastard Shepard save.
There are canon decisions (and a couple based on your gender) that have been made. It's kind of strange because the canon decisions definitely weigh more toward one side than the other.