I only do event rides as I want to ride a different area, on different roads than my everyday. I don't typically join a paceline group as 1) I don't know the skillset of the riders and 2) I usually don't care if I gain 2 MPH on the avg. What might change this if a course has a big wind element and joining the back of a group helps with the wind.
When I did the Bike Tour of Colorado, I would almost never ride in a group, (only once on the last 10 miles into Gunnisom on a flat road). It's very easy to get sucked into a tempo that see's you riding harder than you should, on a ride that has a week of 75 miles per day. You really need to pay attention to effort level on multi-day rides so that you don't burn out. One day events, not as much worry, though I did make the mistake on one of my earliest centuries of hanging on a racing group doing the ride for fun. I was hammered at mile 85.