Agreed.. I forget exactly how far behind you can be + still be in the draft, but there's definitely some room you can leave off the person's wheel without affecting efficiency too much. Also, remember that in the event of a "bump", the guy in the back is going down -- front wheel + fork get turned very easily compared to the fixed/straight rear + inertia from weight on the saddle of the guy in front.

I did a quick google + came up with this... it seems like a reasonable set of guidelines. One of the most important ones I didn't mention before is "not overlapping wheels".. and don't spit/farmer snort unless you're on the back