Ben, if you are attaching a plywood deck bent over beam or bulkhead then glued on sheer strips/clamps then YES you can glass the underside before putting it on, waiting untill the epoxy is tack free makes for less of a handling mess. For near flat or shallow arches you can be less concerned that the epoxy is still soft, for forward deck sections that are one big piece with a significant bend then you should be careful to apply it while the epoxy is just tack free or install the glass on the bias. Laying a damp towel on the topsides will facillitate bending. I installed a 3mmply deck on an 11" radius beam glassed with 4oz on the underside with a teeny stringer (3/16"x1/4"). The top was glassed with 6oz. Without a bulkhead I could stand on the deck with no problem.