Harry Franks has advice on Nulls vs. Lights as Emitters in Particular in Red Giant Quicktip #18:
"Some people like to use Nulls or Solids as emitters in Particular, and some like to use Lights. But when is it best to use one or the other? Both have strength and weaknesses. Here you'll learn where an when to use them. Hey - it's not glamorous stuff, but this is some good Trapcode Particular info that will help you out when you need better control of your emitters."
Red Giant QuickTip #18 : Nulls vs Lights as Emitters in Particular from Red Giant Software on Vimeo.