I'm not comfortable at -1 inches or less. In other words, I avoid wheel overlap. After that it depends on who I am riding with and the condition of the road. Squirrel y riders and rough roads make me move back a bit or take more turns in the front. Usually about 6 inches or so.

I have two cycling buddies that I know and trust so much that we ride in a pace line on aerobars. Looks like a team time trial. We've done thousands of miles like this and only had one incident - which is one too many of course.