My rule is that if I do not feel comfortable riding tight, I will leave enough room to the rider in front of me so I do feel comfortable and safe. Some groups we're in a groove and moving steady and and it feels safe with 6" between riders, other times the speed bounces around a bit, or for whatever reason I don't feel safe on the rear wheel of the rider in front, I simply leave enough room so i do feel safe.
A recent group ride we were on wet roads and the tires in front were kicking up an annoying spray of water from the road. I left about a 12' gap behind the rider in front. A rider behind pulled around me and told me to "come on, stay on my wheel, let's catch those guys". I told him to go ahead, catching them is not a problem, getting blasted with water is.