Don't underestimate their sight! If they are pressured pigs, they will stop often and look around. But, if you have any cover between you and them, utilize it well, and you should be fine if you got the wind in your face.
Like RC says, it helps to cover ground as quick and quiet as you can. They dont usually stay put long, and when they pack up and move to another spot, they can cover some ground quick.
I try to take straight broadside shots. I don't fool with quartering shoot because I want 2 holes in my pig...one on each side. They don't bleed like other critters.