If you are in a defensive lane you still want spray at 1 because it does retarded amounts of damage if they are trying to kill you at level 1 where you have time to spray a lot of time before you go down.
In a passive lane I would probably use this skill tree:
The other team is going to expect you to have high levels of spray because it is standard. It turns out that if you get a significant number of stacks spray still hurts a LOT so if they are diving you then you probably come out even or ahead thanks to the reductions. I haven't done the math but I don't think there would be a significant discrepancy when you factor in EHP which results in more work done by your allied tower and possibly by your allies.