Diagonal placement - penny wise/pound foolish. Get four.
I've never had problems using four. I get it up on all four corners, taking care to watch and feel all four points as it gets let down. Pay good attention. I push on the car to ensure it is sturdy and isn't going to fall over easily.
If jack placement is tight I've never felt it a problem to scoot the jack over some. It is steel, after all. Just be sure it won't slip.
I'm never complacent when working under a car. I'm always as careful as can be. I'm always reading of somebody that let it get the better of him, and I don't wish to be another story in the paper.
As many times as I've cheated the tall guy in the black robes, I don't want to turn around and give him a freebie.