Wow E I was thinking  that yesterday while cleaning a pitch.  I think somehwere in the middle is where you should aim for but it's hard to fight the impulse to really stick on in, so I confess to sometimes sticking it in too far.  My partner claimed she was placing her cams that way because she didn't have enough slings and wanted to reduce the chance rope drag would mess up the placement.  Not sure if I am convinced but in same cases that seems like a good point. 
  
PP