I prefer Go then roll as I prefer to have the player understanding reasons and risks. Then the roll decide if the impact is met or avoided.
It also permit something that I try to do which would be Go, do your things, then roll. It's easier explained with an example: in the case of creating a bomb. Is it a well build bomb or a one which will never be predictable? In fact, the character thinks it's a good one. So I try to let the bomb being built without rolling. When the bomb is planted or ignited, then the player has to make the skill check to know if the bomb was correctly set. We experienced it as more fluid and more dynamic.
I prefer Go then roll as I prefer to have the player understanding reasons and risks. Then the roll decide if the impact is met or avoided.
It also permit something that I try to do which would be Go, do your things, then roll. It's easier explained with an example: in the case of creating a bomb. Is it a well build bomb or a one which will never be predictable? In fact, the character thinks it's a good one. So I try to let the bomb being built without rolling. When the bomb is planted or ignited, then the player has to make the skill check to know if the bomb was correctly set. We experienced it as more fluid and more dynamic.