I just got finished with adding occlusion to all of the content packs I have purchased that use particle effects. As we all know, occlusion is your friend 
I am not entirely sure what the downside is from having it set to “None”… but… yea. Just a friendly reminder from some wanna-be trying to be a game dev type of person.
Are you talking about ambient occlusion ?
Na, talking about rendering occlusion. If it is set to none, even if the particle system cannot be seen (it’s behind something), while it is occluded from view, it will still be rendered. See the below pictures of a before fix / after fix:
Ahh, That looks nifty
Now to find a good use for this… (Amongst about a half dozen other nifty things )