unzipping the window

Back in the "day" we always unzipped back window on our converts before lowering the top. The cars I was familiar with were a '56 chevy, a 65 chevy impala, a 68 chevy Chevelle and a 63 valiant. I was sure surprised when I looked it up in a '63 valiant service manual which said "the rear window need not be unzipped! I checked my '69 Plymouth manual and there are directions for lowering a convert top but there is no mention at all of the rear window...RH