Everything I could reach...
I hand squeezed everything I could reach with a Cleveland Tool Main Squeeze, but that was a small percentage of the 15,000+ the rivets I've set. Since the majority of rivets have to be driven anyway, I didn't see the attraction of a pneumatic squeezer.
I did find that the DRDT dimpler is a really quick and easy tool for dimpling ribs, bulkheads, and stiffeners. Maybe that's why I didn't feel the need for a pneumatic squeezer.