Which has to be declared first: Marklights or Jink?
The reason its important is not for the jinking itself, because you don't "roll" jinking, you are simply given a 4+ cover, which doesn't even come into play until you take potential damage. The defending player can't avoid it, Scour doesn't remove jinking or strip cover saves directly, it simply gives all weapons shooting the "Ignores Cover" rule. It doesn't matter if the jinking player declares it before or after, he ain't getting his cover save. 

This is why you can't get Cover Saves from Markerlights because they don't inflict wounds and you only get cover saves from inflicted wounds/pen or glancing hits.

But its still important because Jink makes you fire Snap Shots. So if the defending player knows he's not getting Cover Saves anyway, he can wait for you to use the Scour ability -2 counters and then let you waste it by not Jinking at all. That way, if the unit survives, it isn't on Snap Shots.

I agree that the word MAY for Jink specifies that anytime within the Select Target steps does the player determine Jinking in effect so long as it is done before rolls are made, even waiting until after the first weapon is selected.

Markerlights are more restrictive, and specify "immediately" before a unit shoots their use must be declared, but how they are expended is declared before the shooting attack, which I agree is before the first weapon is selected (which is part of the shooting attack).

So before you state the weapon you are using, those counters must be used. Markerlight declaration is very clearly before that and the defending player can choose anytime before To Hit rolls.

Tau are my primary army so I'm hurting myself by honorably letting my opponents know before I nominate anything to shoot.