We do now have the technology to implement session specific percentages for the GP league points and Team Points calculations, and it's not terribly difficult to implement.
I would say though that conditions can change not only from one session to the next session, but since a session lasts about 3-4 hours, conditions can change during a session too.
So there's an element of luck involved - it might well be that you're at an advantage or disadvantage based on what lane you start on let alone what session you choose.
My opinion for what it's worth is that you have a free choice in the session you shoot in and you have access to weather forecasts too.
Choosing the session to give you a best chance of a good result comes down to good judgement and could be argued as being part of the 'skill' of the shoot I suppose.
The GP series has been won for the last 2 years by someone who pretty much always shoots in the PM session. If you want to increase your chances of winning - shoot in the PM session.