Generaly, the rule is that the positive option goes in the "more distingshed" position. Whether this is the top, bottom, left, or right mostly depends on who you ask and what the context is. I think, less generaly, when you're in a pick list, like ±, or on PM, the positive options should be first. On a number line, or a plane, the posive options should be in the positive direction of the axis (duh), which by convention is up, or to the right. This is from the european tradition (though I don't know of anywhere it isn't true) that left-handedness is evil... thus "right" meaning both "not left" and "not wrong". The left side signifies the devil and evil, because it isn't normal. Ain't history great?

Anyway, on a pick-list, the best option should be first. On a number line, positive goes to the right.