I set the trigger AFTER I mount the gun. Sometimes I may need to wiggle or squirm a little to get the gun where I want it, and I see no need to have the trigger set while I'm getting into position.
Some shooters set the trigger before or during the mount, but I feel that this method requires a greater amount of grip pressure on the trigger to make sure it doesn't go off while you are getting into position.
Whatever you want your final routine to be, set it just before then.
Logical choice, and what most folks use in games like trap with a shoulder mounted gun, is when the gun is on the shoulder and the eyes are adjusted for where to expect the target, then set the trigger (forget it) and call for the bird. Your routine may vary.
I don't know when I set it. It is not set as I raise my gun but it is set when I call pull. Some things our body does automatically for us. I don't want to think about this question or I will start concentrating on setting the trigger rather than shooting the bird.
After my gun is in position, and I'm comfortable with my mount. Worst thing I do...don't know about you...is to be less than PERFECTLY comfortable with my mount, but go ahead anyhow, rather than taking the gun down and starting over. I set the trigger as I'm moving my focus to the front of the traphouse.