It does matter, as what you want to achieve with a proper trigger pull is a movement which is as close as possible to being exclusively backwards along an axis parallel to the axis of your barrel. You don't want to be imparting any moments of rotation around that axis, or inducing any 'pitch' or 'yaw' (to borrow the aero/nautical terminology).
If you get the chance to operate a really fine shooting instrument with a really good trigger, you will see and feel that proper trigger finger placement and movement reduces the deviation of the sights from the centre of the 'bullseye'. Good target guns even have triggers which are adjustible in three planes so as to limit the deflection imparted by your finger. Ideally, no part of your finger other than the pad of the first phalanx should touch any part of the gun.
Having said all that, what is right for one shooter may not work well for another. Have a look at the
Army Marksmanship Guide, which 'allows' either of your positions a or b. Like anything else in shooting, finding the movement which suits you and your gun* and repeating it consistently is more important than which movement you choose!
* A gun that ill fits your hand, or whose trigger position is not right for your hand, or whose trigger pull is too heavy or jerky may force you to use an unorthodox trigger-finger position or technique. Have a look at Col. Cooper's explanation of using the 'pinch' technique to win a bet in 'The Art of the Rifle'.